$500 for the 210 is good, but make sure it holds water first and really check the silicone - I'd offer him $400 and see if he would come off the price a bit. Its always worth a shot, I just talked a dude down to $300 on a 125, $200 less than what he was asking! $400 max for the 125.
So for the 125 that you bought for 300 what all came with it, did it just include the stand?