Thanks Tony! When one views a listing on the item's description page it does show the number of successful MySlabs transactions a seller has. In terms of a full-fledged feedback system a decision was made (before my time) that since bad actors and folks intent on violating the code of conduct aren't going to remain active members that simply displaying the number of successful transactions suffices to show that an individual is a trust worthy, veteran member.
With regard to returns, we make it clear in our Buying and Selling Guidelines that all individual transactions are subject to the terms and conditions of PayPal and that they have the sole ability to reverse as they see fit. That said we feel that "buyers remorse" and or market fluctuations after a transaction aren't acceptable reasons for a return. As a result we reserve the right to suspend members who choose to conduct business in this way. You're certainly entitled to disagree, but I can tell you with 1000% certainty that the overwhelming majority of our members (sellers especially) genuinely appreciate this and cite the most incredible stories of folks abusing eBay, Paypal, and or Credit-card company return/charge-back policies.