Where I live it is normal for the buyer to select a contractor, have them bid on the work, and then negotiate with the seller about that bid.
I suppose in theory the seller could bring in a competing bid for discussion, but I have not seen it happen.
If you and the seller have already agreed on a price, and your contractor will do the work for that price, why would the seller care which contractor does the job? Usually sellers just don't want to have to deal with stuff they can avoid, and selecting a contractor would be included
.
I'd think you could just name the contractor in the contract, or put in a clause that says you have to approve their choice of contractor (and have your agent suggest yours). Or simply that you will have the place re-inspected after the work is done (also normal where I live). If the job is done right, do
you really care who does it?).