I'm about to buy myself a bicycle but the seller posted there's a slight scratch on the paint. Seems to me it doesn't look like a normal scratch but can be a crack on the carbon itself. Is it a crack or a scratch?
Looks like some scratches. Probably caused by the rear derailleur or chain hitting the chainstay. I’m not sure how it could happen in that location though. Maybe back-pedaling while the chain was stuck somewhere? Or the freewheeling mechanism locked up?
Make sure the rear derailleur and derailleur hanger are straight. The rear derailleur cage should be parallel to the rear wheel and not appear twisted in any direction. Make sure the chain is okay and no links are bent or sticky. Make sure the freewheeling mechanism works and engages immediately and smoothly. Make sure there are no worse scratches around the bottom bracket and rear dropout from a dropped chain.
I don't think you should rely on internet users' analysis of (mostly out of focus) photos. If you are there in person touch the carbon and knock the chain stay to see if it feels or sounds differently from the other chain stay or potentially the seat stays.
Secondly you'll need to make a risk/benefit analysis. For example, a broken chain stay is likely to cause less personal harm than a broken fork. Also, is the price very attractive or is the seller willing to discount due to the uncertainty of scratch vs crack? And, will this chain stay be on your mind every time you get on the bike? These would some questions I'd ask myself in considering this bike.
Lastly, if the bike is priced right and you think it's worth the effort, you could consider having an xray taken by a professional. Then you will know for sure. You could consider asking the seller to split the cost for the xray, instead of a discount.