Don't get the dvla involved from the get go, they honestly don't know their arse from their elbow when things get complicated like this

You've paid for an item

You've received said item

You own said item, with proof of payment and purchase

And now you find said item which is your property, for sale being sold by someone else

That is what you need to explain to plod and then take it from there

Matters of retention and the like are the next step once you've provided evidence that you are the legal owner