Mastercrafts are going to be a full steel frame construction with generous side bolsters to hold you in the seat. As Scout said he runs Mastercrafts and I run Beard seats and will never go back to a stock one. So much more comfortable for wheelin and hell just sitting. The way they will mount on your stock slider brackets will give them plenty of lean.
Did the owner state whether they had been installed in a TJ already? If so then just un-bolt yours and bolt his in. The tear in the cover isn't a problem. All after market seats can be re-covered pretty cheaply considering that you are probably getting about 600 worth of seats for nothing.
Take a look at them. What can it hurt?