I have had the chance to buy online even before the invention of eBay i.e. circa 1994.
Over the years, the accepted practice at least in the US goes something like this:
Mint -New or new old-stock items that have never been owned and could then, be mint.
10 (or 10+!) - items which have been owned , are in exceptional condition ... and could indeed pass for mint.
The rule seems to surround the question of ownership. So, as long as an item is owned... it can never be mint. At best, 10+. 
For new items that are new old-stock and in non-mint condition... the term used is "shop-worn" or "shop-soiled".
On the other extreme, among local sellers there are people who sell pre-owned items while indeed mint, are priced 10% below new. In some instances... that difference is effectively the cost of transport to go meet the seller!!
Again, the rule-of-thumb is that any item that is pre-owned and in reasonably good condition should be priced at least 30% less than new to be a reasonable deal. 
Only in Singapore, I make an exception at 20% because really, some of the resale pricing really borders on the ridiculous.
KEH in the US has a fairly good grading policy, and have earned themselves a reputation for better-than-expected as they grade conservatively.
But as this shows , it's still all very subjective. The best is for the buyer to determine what standards he/she can live with, ask the relevant questions ... and ultimately caveat emptor. Sometimes if it sounds to be too good a deal, it really is.