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.