Marin frames from that era are more prone to developing cracks. Make sure to inspect it thoroughly, especially at the bottom bracket.

Seeing things like the front QR skewer improperly tightened, chain not even touching the freewheel/cassette, rear brake unhooked, faded anodization on the fork frown, etc. tells me that this is a project bike that will likely need a fair bit of work to get up and going. If you want it just to ride around, $15 is likely a good deal if the frame isn't cracked. If you want to use it as a mountain bike, I'd keep searching for something higher quality.