That thought did occur to me. The only item I purchased used was a midspan since my switch isn't PoE, but it's a high-end (albeit used) unit I got very cheaply; midspans seem to have no resale. However, that doesn't explain why each one locked up only once over the course of less than a week, unless it's simply coincidence.
There's no such thing as coincidence in this game!
So your using local mid spans to supply power, I doubt if it's the switch then but it's one less thing out of the chain, how about spikes on the local mid spans supply? Could cause a camera to lock up. Look for high current devices on the same circuit.