This is a basic contract issue governed by the local state law. Most states follow what is known as the Uniform Commercial Code. There is an enforceable contract here. However, under 2-401 of the UCC title passes based on when the contract says it does. If the contract is silent on this, and the watch was to be delivered to you by the seller, then title passes upon completion of the seller's performance (I.e. upon delivery or turnover of the watch, not upon the signing of the paperwork and payment).
In this case, the seller breached the contract by failing to deliver or tender the goods to you. As a generally matter, there is no right to specific performance (I.e. right to compel the turnover of the goods) unless, pursuant to UCC 2-716 the goods are considered to be so unique so as to create such an entitlement. Getting a court to order specific performance is not easy. The item would have to be very unique (and very, very hard to replace). If a court deems specific performance to be unavailable to you, you do have a right to monetary damages for the breach of the contract (which in this case would be the value lost to you if you could in fact prove that the watch was worth more than the contracted for amount). All this, as a practical matter, is not very satisfying I'm sure. It is a situation that cries out for a negotiated solution with the seller, perhaps under the threat of disclosing his/her identity (aka a bad review).