Due diligence is the answer. Read a seller's feedback. Put a good eye on the scan. Ask questions here on the board. PM people you trust and respect for their opinions. Read the threads where people express their public opinions. This hobby is best enjoyed with a modicum of caution but everyone's experiences differ. Tolerances for card condition and acceptable shipping times vary. To list sellers, good or bad, is a short cut with glaring consequences. Who to include or omit would be a fulltime headache that the moderators here do not need. We are all adults. Transactions, positive and negative are bits of data to be learned from, first-hand. There is no instant, painless alternative to years or, hell, decades of time in this hobby. Newbies need to, "Step lightly but keep a steppin'. ".
I shall now step down from the soap box.
