To be honest I'd just buy a replacement seal and fit it myself, I mean you say you bought it to play with, so unless you know exactly what you are doing ( I don't, believe me) then you are going to break it, I've managed to break a whole heap of watches in the past self teaching myself watch repair, I wouldn't lose any sleep over a missing case gasket, let the seller know if you must but personally I wouldn't really bother.
I had a 7548 7000 from the bay, that was recently "serviced", upon receiving it the movement looked like it had been deep fried. Everything was OK, dial and hands not ruined by it. Just let the seller know that it was in bad way, other things were wrong, but the price was very good. I sorted them all out at no extra cost, had spares to hand. Let them know of the problem, but if your happy with the watch, no point rocking the boat, even ask them to send a gasket or two as good will?
I would just pop a seal into it. As long as it wasn't overpriced to start with it's not that big of deal. It's entirely possible the guy just didn't remember to put it back in. I've gotten a few off Ebay with no case seals either, it happens.