Have you considered constructing a
collapsing floor? If you build a depot on a constructed floor, and detach it from any support, you could have it plummet to the bottom of your pit. However, any creatures under the construction as well as most items will be destroyed. Another drawback would be the need to reconstruct the platform every time.
I do know that a cave-in can cause nearby creatures to be flung about, but I'm not sure if you can do that with consistency. If it is possible, you could have a section of wall collapse near the depot and have the shockwave/dust cloud throw them off into the pit, but I don't know if that can work.
If you're not trying to keep a particular hole size and don't mind making it bigger, you could construct a pathway that has a floor consisting of retractable bridges, and pull a lever when merchants come.
Unfortunately, I can't really think of a more elegant solution to this problem.