I'm just starting a saddle search, and from a buyer's perspective, I'm hesitant to spend that much money on something without a return option- there are just so many variables that it's impossible to know if a saddle will work without putting it on my horse. When buying from someone on the internet, there's the additional risk that the item might not ever reach you at all- either because the seller took the money and ran or because it got lost in transit.
If the buyer sends the money in full through Paypal, I believe the Purchase Protection policy applies if the item is never received, isn't as described, etc. but doesn't necessarily apply if the item is as described, but just doesn't fit. (Although it might still apply if the Paypal invoice indicates a return period that wasn't honored) If you use your credit card, most credit cards offer additional protections, but then Paypal takes a cut of the sale. In this case, it makes sense for the buyer to pay in full up front (which also discourages people from taking the saddle on trial on a whim)
As long as I got at least a couple days to try it out after receiving the saddle, I'd personally be willing to pay in full up front for a saddle through Paypal with my credit card, as long as the seller marks the terms of the return period on the invoice (how many days from receipt of goods, who pays shipping both ways, etc.). I wouldn't expect a lengthy trial period from a private seller.