From eBay look at the feedback, not just the numbers (and 98% is not particularly good) but also the actual remarks. You can read a synopsis of negative and neutral FB by clicking on the numbers.
This will also help you to understand if the seller has unhappy customers or if he has recently sold to a random nut on a tear. It happens.
Look at the DSRs. Particularly look at the Item As Described DSR. The usual Rating is 5 Stars. At 4.3 Stars, a seller's ability to list and to sell is severely restricted.
From your viewpoint as a collector.
Buy the **bleep** catalogue. It will save you a fortune over time.
Read the **bleep** catalogue. It will allow you to identify fakes, forgeries and cinderellas , and understand the difference, on sight.
Look for scans not photographs. They are more detailed and can be enlarged for a detailed look.
Ask the seller questions.
And of course, just as at the mall, if it sounds too good to be true, it probably is.
