Generally anything that has been fairly decently maintained will have few issues.
Expect to pull it apart for a clean.
Anything by say Eclipse for the last 5 plus-ish years would have spares readily available.
People got stuck up on shot counts a while back. General wear items are orings / reg seats / detents that are cheap and readily available. As long as paint / water has not got all over a board or solenoid they tend to last decades.
As long as you don't pick something up that does not have the ability to be programmed to current rule sets you have quite a range of markers / ages available