I've used Tacswap before, treat it like any other classified section. Their are Scams on there, as long as you're careful you should be all right. I've purchased multiple things on there before and have yet to have an issue *knock on wood*.
As far as purchasing a used Aimpoint, make sure that it is not under a Military serial number. If the serial number says it belonged to the Military, and you're not using a Military email address they won't fix it. I had a Comp M4 I picked up used, I had a AA battery coorode in the battery compartment. Aimpoint wouldn't touch it, and I was left with a $500 paperweight. When purchasing an Aimpoint now, I either look for non Mil models, or I want the box etc.