I think Rob Lifson's explanation for authenticating autographs and why REA uses JSA is worth a read for anyone who is interested in collecting signatures that they did not obtain themselves. You can read it here:
http://blog.robertedwardauctions.com/?p=83
I specifically found the reason they use JSA as informative, and have pasted that section below:
At REA, we utilize the services of James Spence Authentications (JSA) to review all signed items. We think very highly of their service. This doesn’t mean that all other autograph authentication services are not good. This is what we have chosen to do because we believe that JSA offers the best authentication service in the business. That doesn’t mean that they are perfect, that they have never made a mistake, or that their opinion could not change on a given item. Some other services and individuals may be very good at providing authentication services. We find that almost 100% of the time (not 100%, but almost 100%) that if a signed item is consigned to REA with a letter from certain companies (such as PSA) or certain individuals, that JSA has the same opinion. We also find that when items are consigned with letters from certain other individuals and companies, approximately 100% of the time JSA is not OK with these items. Obviously, this is not a coincidence. Statistically, that would be impossible. When a signed item is consigned that is already accompanied by a letter of authenticity, the item and letter are separated by us before being presented to JSA for review. At REA, JSA has no idea if an item has previously been reviewed by any other service before being presented with the item authentication review. We have even had cases (very rare, but it has happened) when JSA has told us they are not comfortable writing a letter on an item previously authenticated by JSA. This is very rare but knowledge increases over time, and opinions can sometimes change over time.