For godan verbs like 売る, its potential form and passive form look different.
- 売る: dictionary form, "to sell (something)"
- 売られる: passive form, "(something) is sold"
- 売れる: potential form, "can sell (something)"
売られる almost always means "is sold", not "can sell" nor "can be sold". (Strictly speaking, there are a few old people that still use 売られる for the potential meaning, but it's fairly rare.)
"can be sold" is passive-potential (the combination of potential and passive), which is tricky to express in Japanese (see this). Depending on the context, you can say either 売られうる or 売られることができる, but it's usually better to rephrase it to the non-passive equivalent. If 売れる or 売ることができる works for you, you can just use it.