For me it is simply a matter of time. It takes time to compile and report to GPA. GPA can data mine the auctions for sales info. With static sales through my web-site, storefront, or at shows I would have to put the sales info together and send it to GPA in a format that works for them. It is actually probably a pretty easy thing to do. But it is one more thing which I would have to do...on top of running shops, paying employees, negotiating leases, grading comics, bidding in auctions, posting inane mess on comic chat boards, and having a personal life. I just don't want to take the time to report sales to GPA.
You'd think they'd offer to set it up FOR you since it benefits them more than you.