its a tricky one, unless it's been skirmished it's likely (but by no means certain) that it hasn't seen much use/abuse.
thing is, external appearance=/= performance
take my m4, when i got it it was plainly a gun that had been upgraded by someone who wanted a good performer, then ran into the ground by god knows how many owners, it was shocking on the inside. the wiring was a borderline dangerous patchwork of people who's soldering skills ranged from merely bad to "**** it twisted together with a bit of tape is fine", the gearbox internals were shot, bearings on the verge of exploding, gears worn down, the whole thing was a mess.
but now, despite looking pretty much the same externally to a casual observer it's had a whole new box, etu with complete re-wiring, new motor, new hop unit, new barrel and a lot of finnicky tech work to get it just so (well, apart from the ludicrous fire rate that means i don't wanna risk full auto but that was a side effect of going for crazy good trigger response in semi) and it's a completely different gun to shoot.
generally my rule is i won't buy something secondhand unless i can shoot it first, which pretty much limits me to in-person sales. only thing i've bought online secondhand was an nv scope on here, but that was mainly for price and knowing it's a simple immediate test to determine if it was working as described or not.
that said, this hobby is a bit like rc, signing for the package instantly knocks the price down.