CJ, that the movement is COSC certified doesn't mean much when you buy a second hand watch that is a decade old.
If the watch had been regularly serviced before ending up at that retailer, then yes you may expect chronometer specs. But if that hadn't been the case, you should lower your expectations and budget for a servicing.
Again, that's speaking from hand-on experience acquiring my 2503. It's in dire need of a maintenance, losing at least 30 seconds per day.
But guess what : I've never thought of returning it - not even once !
It's a first generation AT which came without papers, so given its pedigree (or lack thereof), I was always aware that I couldn't reasonably ask for guaranteed good specs. It's a shortcoming, yes, but one that can be fixed... whenever. I keep enjoying to wear a watch that really spoke to me, and which I acquired through a fair deal.
So my advice to you is that you shouldn't be hung up on timekeeping performance - as long as that model sings to you and can be had in a good condition and at a reasonable price
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