I have bought a large number of used guns online from the sources you mention, as well as fron private individuals with listings both here and on GlockTalk. I ask questions, get a feel for the seller (and the gun) and only go for the deal if my spider sense gives me the green light. I've yet to get burned (knock on wood) and have been very pleased with every transaction.
Since the first of January this year the supply of new handguns here in Maryland has virtually dried up due to the communist law requiring every new handgun to have an internal lock (limits you to Smith & Taurus revolvers, Springfield 1911s and a small handful of others) - so I've been buying and "importing" lots of quality used guns into the Socialist Workers Paradise of Maryland. IMHO the ability to buy used online has been "berry berry good to me."
YMMV