I think it's a combination of veteran sellers' review knowledge and the high current grading costs. Veteran collectors/sellers know what to look for RE centering and surface (which in my opinion are the two sub-grades amateur collectors fall down in with regards to submitting cards for grading -- edges and corners are much easier to evaluate). RE grading costs... PSA's lowest tier is currently $100, SGC around $30, CSG $20 if you don't want to wait 8 months, and BGS...well I am not sure what's going on with them. And those costs don't include outgoing postage, insurance, etc. So personally, I can understand a long-time seller who is willing to let a raw card go at say $25 even though a graded gem might sell for a lot more down the line.