Sorry, I know only workrelated examples so I don't know if it can be compared to your question about sponsored reviews
I have a client owning a publishingbusiness for professional photographymagazines.
Whenever a new camera, lens,printer ect is launched, they get the item to test it and review.
Sometimes they can keep the stuff just to say tnx for their time.
A friend of mine writes for computermags, same story there.
There is a Professional Photographyacademy here where a few studyrooms are totally sponsored by the big brands, ie Canon,Nikon ect. Their name is on all the walls and that means the students will by the brand more easily, that's what marketing is all about isn't it?
In your case I am sure the review would have been the same if the dealer send you the watch without you knowing his/her name but would that be interesting for the dealer? I don't think so.......
Would be totally unbiased though if the dealers send watches for review to the admin for example and he /she would send it to the reviewer anonymously?