This appears fine, but I'm not quite sure about library cases.

code/game/objects/structures/displaycase.dm

@@ -10,7 +10,8 @@
 	max_integrity = 200
 	integrity_failure = 0.25
 	var/obj/item/showpiece = null
-	var/obj/item/showpiece_type = null //This allows for showpieces that can only hold items if they're the same istype as this.
+	//var/obj/item/showpiece_type = null //This allows for showpieces that can only hold items if they're the same istype as this.
+	var/list/compatible_items_typecache
 	var/alert = TRUE
 	var/open = FALSE
 	var/openable = TRUE
@@ -133,14 +134,14 @@
 				to_chat(user,  "<span class='notice'>You [open ? "close":"open"] [src].</span>")
 				toggle_lock(user)
 	else if(open && !showpiece)
-		if(!istype(W, showpiece_type) && showpiece_type)
+		if(compatible_items_typecache && !is_type_in_typecache(W, compatible_items_typecache))
 			to_chat(user, "<span class='notice'>This doesn't belong in this kind of display.</span>")
 			return TRUE
 		if(user.transferItemToLoc(W, src))
 			showpiece = W
 			to_chat(user, "<span class='notice'>You put [W] on display.</span>")
 			update_icon()
-	else if(istype(W, /obj/item/stack/sheet/glass) && broken && glass_fix)
+	else if(glass_fix && broken && istype(W, /obj/item/stack/sheet/glass))
 		var/obj/item/stack/sheet/glass/G = W
 		if(G.get_amount() < 2)
 			to_chat(user, "<span class='warning'>You need two glass sheets to fix the case!</span>")
@@ -250,12 +251,22 @@
 /obj/structure/displaycase/captain
 	start_showpiece_type = /obj/item/gun/energy/laser/captain
 	req_access = list(ACCESS_CENT_SPECOPS) //this was intentional, presumably to make it slightly harder for caps to grab their gun roundstart
+	var/static/list/captain_typecache = typecacheof(/obj/item/gun/energy/laser/captain)
+
+/obj/structure/displaycase/captain/Initialize()
+	. = ..()
+	compatible_items_typecache = captain_typecache
 
 /obj/structure/displaycase/labcage
 	name = "lab cage"
 	desc = "A glass lab container for storing interesting creatures."
 	start_showpiece_type = /obj/item/clothing/mask/facehugger/lamarr
 	req_access = list(ACCESS_RD)
+	var/static/list/rd_typecache = typecacheof(/obj/item/clothing/mask/facehugger)
+
+/obj/structure/displaycase/labcage/Initialize()
+	. = ..()
+	compatible_items_typecache = rd_typecache
 
 /obj/structure/displaycase/noalert
 	alert = FALSE
@@ -268,9 +279,11 @@
 	var/is_locked = TRUE
 	integrity_failure = 0
 	openable = FALSE
+	var/static/list/trophy_typecache = typecacheof(/obj/item)
 
 /obj/structure/displaycase/trophy/Initialize()
 	. = ..()
+	compatible_items_typecache = trophy_typecache
 	GLOB.trophy_cases += src
 
 /obj/structure/displaycase/trophy/Destroy()
@@ -371,12 +384,17 @@
 	density = FALSE
 	max_integrity = 100
 	req_access = list(ACCESS_KITCHEN)
-	showpiece_type = /obj/item/reagent_containers/food
+	var/static/list/kitchen_typecache = typecacheof(/obj/item/reagent_containers/food)
+	//showpiece_type = /obj/item/reagent_containers/food
 	alert = FALSE //No, we're not calling the fire department because someone stole your cookie.
 	glass_fix = FALSE //Fixable with tools instead.
 	var/sale_price = 20
 	var/datum/bank_account/payments_acc = null
 
+/obj/structure/displaycase/forsale/Initialize()
+	. = ..()
+	compatible_items_typecache = kitchen_typecache
+
 /obj/structure/displaycase/forsale/update_icon()	//remind me to fix my shitcode later
 	var/icon/I
 	if(open)
@@ -395,13 +413,13 @@
 	return
 
 /obj/structure/displaycase/forsale/attackby(obj/item/I, mob/living/user, params)
-	if(istype(I, /obj/item/card/id))
+	if(isidcard(I))
 		//Card Registration
 		var/obj/item/card/id/potential_acc = I
 		if(!potential_acc.registered_account)
 			to_chat(user, "<span class='warning'>This ID card has no account registered!</span>")
 			return
-		if(payments_acc != potential_acc.registered_account && payments_acc)
+		if(payments_acc && payments_acc != potential_acc.registered_account)
 			to_chat(user, "<span class='warning'>This Vend-a-tray is already registered!</span>")
 			return
 		if(!payments_acc && potential_acc.registered_account)
@@ -410,41 +428,42 @@
 			to_chat(user, "<span class='notice'>Vend-a-tray registered. Use your ID on grab intent to change the sale price, or disarm intent to open the tray.</span>")
 			return
 		//Buying the contained item with the ID.
-		if(user.a_intent == INTENT_HELP)
-			if(!showpiece)
-				to_chat(user, "<span class='notice'>There's nothing for sale.</span>")
-				return 1
-			if(broken)
-				to_chat(user, "<span class='notice'>[src] appears to be broken.</span>")
-				return 1
-			var/confirm = alert(user, "Purchase [showpiece] for [sale_price]?", "Purchase?", "Confirm", "Cancel")
-			if(confirm == "Cancel")
-				return 1
-			var/datum/bank_account/account = potential_acc.registered_account
-			if(!account.has_money(sale_price))
-				to_chat(user, "<span class='notice'>You do not possess the funds to purchase this.</span>")
-				return 1
-			else
-				account.adjust_money(-sale_price)
-				if(payments_acc)
-					payments_acc.adjust_money(sale_price)
-				user.put_in_hands(showpiece)
-				to_chat(user, "<span class='notice'>You purchase [showpiece] for [sale_price] credits.</span>")
-				playsound(src, 'sound/effects/cashregister.ogg', 40, TRUE)
-				icon = 'icons/obj/stationobjs.dmi'
-				flick("laserbox_vend", src)
-				showpiece = null
-				update_icon()
-				return 1
-		//Setting the object's price.
-		if(user.a_intent == INTENT_GRAB)
-			var/new_price_input = input(user,"Set the sale price for this vend-a-tray.","new price",0) as num|null
-			if(isnull(new_price_input) || !user.canUseTopic(src, BE_CLOSE, FALSE, NO_TK))
-				return
-			new_price_input = CLAMP(round(new_price_input, 1), 10, 1000)
-			sale_price = new_price_input
-			to_chat(user, "<span class='notice'>The cost is now set to [sale_price].</span>")
-			return 1
+		switch(user.a_intent)
+			if(INTENT_HELP)
+				if(!showpiece)
+					to_chat(user, "<span class='notice'>There's nothing for sale.</span>")
+					return TRUE
+				if(broken)
+					to_chat(user, "<span class='notice'>[src] appears to be broken.</span>")
+					return TRUE
+				var/confirm = alert(user, "Purchase [showpiece] for [sale_price]?", "Purchase?", "Confirm", "Cancel")
+				if(confirm == "Cancel")
+					return TRUE
+				var/datum/bank_account/account = potential_acc.registered_account
+				if(!account.has_money(sale_price))
+					to_chat(user, "<span class='notice'>You do not possess the funds to purchase this.</span>")
+					return TRUE
+				else
+					account.adjust_money(-sale_price)
+					if(payments_acc)
+						payments_acc.adjust_money(sale_price)
+					user.put_in_hands(showpiece)
+					to_chat(user, "<span class='notice'>You purchase [showpiece] for [sale_price] credits.</span>")
+					playsound(src, 'sound/effects/cashregister.ogg', 40, TRUE)
+					icon = 'icons/obj/stationobjs.dmi'
+					flick("laserbox_vend", src)
+					showpiece = null
+					update_icon()
+					return TRUE
+			//Setting the object's price.
+			if(INTENT_GRAB)
+				var/new_price_input = input(user,"Set the sale price for this vend-a-tray.","new price",0) as num|null
+				if(isnull(new_price_input) || !user.canUseTopic(src, BE_CLOSE, FALSE, NO_TK))
+					return
+				new_price_input = CLAMP(round(new_price_input, 1), 10, 1000)
+				sale_price = new_price_input
+				to_chat(user, "<span class='notice'>The cost is now set to [sale_price].</span>")
+				return TRUE
 	if(I.tool_behaviour == TOOL_WRENCH && open && user.a_intent == INTENT_HELP )
 		if(anchored)
 			to_chat(user, "<span class='notice'>You start unsecuring [src]...</span>")
@@ -463,7 +482,7 @@
 		to_chat(user, "<span class='notice'>[src] must be open to move it.</span>")
 		return
 	if(istype(I, /obj/item/pda))
-		return 1
+		return TRUE
 	. = ..()
 
 /obj/structure/displaycase/forsale/multitool_act(mob/living/user, obj/item/I)