My suggestion is to have an optional redstone OUTPUT from trade signs. Things this could be used for, and a more detailed description are in the below quote from a recent suggestion on the same topic.

If you still don't understand, basically, when a tradesign is clicked and you pay X amount of money, it will activate a redstone current behind the trade sign, like a button. It would basically be a button that you pay to use. This has been suggested MANY times in the past and most of the threads have simply died out. If you have any questions or comments about this, please post below.


EDIT: It doesn't have to be necessarily a trade sign. A good idea from @lapkingkn is

 

It would be like a button, so it would go away after a second or whatever it is (unless its modifiable to make it last for like 5). If one person pays for multiple people, that is smart of them, you cant really do anything about that.

Also, I just had a new idea. Maybe, if trades signs would be buggy or whatever, make a new sign like

[RedstoneO]
*cost*

 

*has already had those ideas*

;)

 

I don't know much about programming, but I think it would be something simple like on useage: activate redstone