The shop plugin we use originally used block IDs instead of punching the sign, like you suggested. They modified the code to create the shop system we have now, which I prefer much more than the ID system.

Your suggestion would help a bit with acquiring items you don't have, but you'd have much better luck finding those items by asking in chat/Discord/these forums. The number of people who see your post in those three mediums is much higher than the number of people that will come to your island and see that you're trying to buy something. If your concern is that you may not be online when the other person wants to trade, I can understand that. I still think it'd be best just to orchestrate through Discord some time you both could be online.

Side note, your suggestion seems formatted wrong.
With our current system, the sign would be:
[trade]
1
100
trade
You'd then borrow the head you want to buy, punch the sign with it, then punch the sign with diamonds.

Merge of our current system with the original plugin would be:
[trade]
1 397:1 (if asking for wither skull) or minecraftlayer_head{SkullOwner:"MHF_<Name>"} (if asking for a player's head)
100
trade
You'd then punch the sign with dias, and you'd have a trade up, asking for 1 head for 100 dias.

Full revert to original plugin would be:
[trade]
1 397:1 (if asking for wither skull) or minecraftlayer_head{SkullOwner:"MHF_<Name>"} (if asking for a player's head)
100 264 or minecraft:diamond
trade
Then the trade would just work.

 

Here is the plugin they got the source code from: TradeShop | SpigotMC - High Performance Minecraft

(At least these are all my assumptions. I don't have access to the Skyblock Git, so I'm not sure exactly what their base for all of their plugins is. This plugin was just the most identical plugin I could find to what we have)