How to buy right
Know the reference before you see the watch. Know what the dial should look like, what the caseback should say, what a correct bracelet feels like. Arrive knowing more than the seller expects.
Ask about service history. Ask whether any part has been replaced. Ask whether the case has been polished. Ask to have the watch independently authenticated before you complete the purchase. A seller who objects to any of these questions is a seller to walk away from.
Get a receipt that identifies the watch by reference number and serial number, describes the condition as represented, and records any specific claims the seller made. That documentation is your recourse if something is wrong.
CollectorGrade take
The best time to do due diligence is before you develop emotional attachment to a specific piece. Once you have decided you want it, objectivity becomes harder. Build your verification process into your buying routine before you start viewing, not after.