To me this seems to be simple. There are two parts to this. The formation of the contract and the performance of the contract. It would appear that the contract was formed when the offer to sell at an item at a particular price was met with an acceptance of the price and an agreement to pay. Absent of fraud, misrepresentation or other substantive deficiency, the "consensus ad idem" principle is satisfied. This is evidenced by the documentation. At that point the contract is formed and both parties have rights and duties.
The performance of the contract then comes into play. The seller has the obligation to deliver the item and the buyer has the obligation to pay the price (if not already done). If either side fails to perform their obligations then the other has the right to claim breach of contract and at least withdraw from the contract (damages/restitution claims may then be applicable). However the other route is to insist that the non-conforming party adheres to their obligations under the contract. In this case, it's either payment of the agreed amount to the seller or the seller delivering the item. Once again, it may need legal intervention to enable one party to enforce their rights under the contract on the non-performing other party.
In my opinion, neither buyers nor sellers "remorse" is a valid reason to back out of contracts. There is not a special set of rules that apply to watch collectors and dealers apart from those applicable to everyone else.
Of course. all the above is a theoretical analysis, and in the end, whether either party wishes to insist on their rights under a contract must be weighed against the potential cost of doing so (with or without formal legal assistance and cost).