Can't sell items sometimes?

I'm really confused about the pawn shop mechanic. I've read that selling elixers was a good way to make money, but I was only able to sell one before it suddenly turned into a blank line instead of the green sale price. I have this same issue selling fish, so I don't think it's just an exploit that was patched out. I sold an octopus for around 120, but then couldn't sell another one. I lost a few items doing this before I realized he was just taking them from me without paying. Is this a bug? I want to clear out my storage, but I was hoping to get a little extra cash from it.

Edit to add that I'm playing on switch