Would you consider the anonymous sale of nudes cheating?

I think the fact they're asking shows they're not sure.

But also, one should be sure what your partner constitutes as cheating before engaging in said action, yeah.

My wife and I do not see the selling of nudes as cheating (unless non-consent to the nudes or the distribution, then it's a lot of levels of gross) but others might disagree.

Yep, you're asking everyone except the right person, here. Ask your partner. If you're afraid to ask your partner, then assume it's cheating until you do.