Wow, I saw 2 new pages in the thread and thought "yay! New seasons cards posted!"...
Trading post feels pretty near to strictly superior to a few cards, being terminal on play only marginally balancing the advantage it has over Treasury, in my opinion.
I think it needs another handicap that would make it more unique and give the other poor Peddler variants a chance to compare favorably.
As a random thought: "while this is in play, if you buy a treasure, trash this" (or discard, but I prefer trash as it's less Treasurish).