A new shop plugin has been added in preparation for the upcoming Minecraft 1.8 update. Currently Duckshop is still active and usable, but convert your shops over to SignShop as you have opportunity. A page has been added to our wiki covering usage and creation of shops.

I am sorry about the inconvenience this may cause. Unfortunately I have reason to be certain Duckshop will break at 1.8, as it is no longer maintained by the developer and depends on the number IDs that Mojang is removing in 1.8. So the plugin will be removed when Pearl is updated or when there's no more active Duckshop signs left, whichever happens sooner.

Best regards,
~Dragoness

 

I'm not entirely sure what you mean by a single transaction at a time.

Technically, there are some other kinds of shop plugins we could try out. SignShop simply is one of the top alternatives to Duckshop, among those highly likely to continue being developed.