The sheep's corpse doesn't show up directly in the trade window. You would have to place it in another container to be able to sell it.
Did you talk to Bertia about her stolen sheep? That will give the corpse to her.
Do you have a recent save you can reload?
The Save Game Editor
may be able to transfer the sheep's corpse from an old save to a new one.
You can email email@example.com, with a description of the problem and the report.zip file generated by the D:OS support tool
(for the PC version). The report will contain system and game information, as well as your saved games. To reduce the file size, all but your latest save can be deleted from the zip.