Personally I try and stick to reputable companies like ffordes,
park camera's, misfuds, etc that you have some guarantee from. But the same as with most
second hand good, try before you buy, is the item as described by the seller, if you can't see it, ask for lots of images, check for scratches, marks, signs of use, for lenses (especially sigma) the paint comes off so not so worried about that, but things like, is the lens optically sound, take a card and take some images, again fair signs of use, any scratches on the lens that could effect optics, basic comment sense when buying secondhand goods. Does it look like someone looked after it or does it look like a dogs dragged it through a hedge backwards.
My 1D was a reconditioned body, and just bought a 2nd hand 300mm f2.8, both cost considerable £'s, but very happy with purchase and saved mega ££'s of new prices for nearly brand new gear.