Bringing this back to life since it was brought to my attention.
There's a ton of grey area here - technically speaking, the texture item and the texture itself are separate things.
However, practically speaking, people really should be intelligent as was said and put dummy textures on items that they sell. However, someone else's mistake does not necessarily mean your ownership. For one thing, what if the item has stolen textures on it? There's no way you can claim ownership in that case. For another, plagiarism cases are very rarely investigated to that extent initially, so it's very likely that if they reported you, you'd get banned.