I bought 6 regs off e-bay, 2 brand new apeks (from a brick-and-mortar dive far from me) and 4 used scubapro mk10 with g250 or g295 seconds. All were professionally examined and rebuilt as needed. The new ones needed nothing... the 4 used regs are set up as pony/stage bottle regs. All have worked well. I also have two, bought new, scuba pro regs (mk 25, g600 combos with secondaries). This provides a complete setup for me and my gf, including ponies, stages and a spare..
None of the used units cost more than $80. (1st & 2nd stage), with the shop work and shipping, about $125 each. Got bunch of weird bits along with the used regs -- almost worth the purchase in humor value. Things like a reg spliced into the bc inflation hose (yes another reg worthless to me), fancy painted covers, and valve and flex gizmos.
Regs are one of the items i actually think e-bay is good for, as a competent tech can fix or declare unsafe with high accuracy (assuming a good tech). I worry more about rubber goods (where deterioration can be missed more easily), bcs (with hard to check bits), and wet and dry suits (where fit matters)