At 21" that's likely a 17" seat, since it looks like the cutback is 3". For seat size on a cutback saddle you need to measure from the cutback to the cantle, not from the front of the saddle.
Saddleseat saddles are difficult to sell because they're not really in demand, and ones made from Argentine leather will be even less desirable. Saddles made from Argentine leather are poorly made and don't hold up well.
I have a Crump saddleseat that I've had for 30 years. It looks almost brand new, even though it was the primary saddle I used for many years. Yes, even trail riding.
I'd be reluctant to pay more than $50.00 for that saddle, especially since you want $35.00 for shipping. It's not worth very much, and doesn't look to be in the best shape.