Let me see if I've got this right,
Old guy walks into Rolex AD, buys new Submariner and trades in old Submariner - which the OP subsequently buys (quickly), sends it straight off for a full fettle and we all go - ooh ahh, but wouldn't the old Sub be worth more than the new?
Wouldn't the AD, have had the old Sub fettled themselves for a profit, or advised the old dude accordingly?
If I'm besmirching the good reputation of the OP and he was simply the benefactor of a generous - nay philanthropic AD then apologies.