That feels super strong to me anyway. Nearly-as-good-Chapel (already stronger than the usual $2), then add an Action AND a Buy AND some money? Never mind Chapel -- it blows Trading Post out of the water:
* Trading Post is terminal.
* Trading Post doesn't give +Buy.
* Trading Post requires trashing two cards, rather than allowing you to trash 0, 1, or 3 cards instead.
* Trading Post effectively gives you +$2. Your card matches Trading Post when you trash an equal number of cards, but it can do better if you want, and allows you to do worse if you'd prefer.
* Trading Post gains a Silver. This is the one thing that makes Trading Post better. But gaining a Silver on its own is a sub-$2 effect (see Squire).
So I'm kind of surprised this shouldn't be $6 or nerfed. That said, I would suggest that a trasher priced correctly before Dark Ages is still correctly priced after Dark Ages. I mean, the other official trashers didn't suddenly change prices, so if you price a trasher by comparing it with the official trashers, you'll still be in the right ballpark.
In a nutshell, trashers + on-trash-effects = appropriately strong combo, not something game breaking.