The first serious watch is usually bought with the heart and regretted with the head. Three things are worth checking before anything else: proportion on your wrist, the integrity of the bracelet, and the completeness of the set.
Proportion is personal — a 41mm Submariner wears larger on a 16cm wrist than the photographs suggest. Bracelet stretch tells you how the watch was worn; a stretched Oyster bracelet is expensive to correct. And papers matter: a full set can be worth 15–20 per cent more than a watch alone.
Finally, buy the seller. A five-figure watch from an unknown source is a five-figure risk. Ask for the inspection report, the provenance and a return window — and walk away from anyone who hesitates.