It's nothing wrong (assuming you have permission!!) with posting photos of other peoples fish. However, it should be made clear that it is not your own fish - for a variety of reasons:
1. It makes it clear why you are asking (in this case, "I'm planning to buy these, can you please sex them").
2. If someone else sees the picture of a fish for sale, they may either think you are the same person as the one selling the fish, or, worse, think that the seller is selling fish with your picture (it has been known that people advertise fish they don't actually have, with pictures of other people's fish).
The seller may also think it's rather rude to question their statement as to what the fish is (or the sex, etc). The way the post is presented can either make this worse or better...
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Mats
