I found an used Skywatcher 70mm telescope for buying, but its lenses are a little suspect about harming. In one photo, it looks clearly a scratch, but in the another, it is not visible. Could it be an optical illusion or is it really a harm? If it's a scratch, should I discard the possibility of buying?
Price vs. quality. You're unlikely to see any significant degradation in the final image if those are scratches. However, that is not how you should make your purchase decision. You want to inspect the entire body for any evidence of dents, bends, etc. which indicate rough usage, not to mention misalignment of the optical elements.
If you can't try out the 'scope in person, make sure there's a 100% refund guarantee before having it shipped to you.