Actually Central, I sell very few 'used' games (no used DVD's). I have tried in the past but the good money (and feedback) is in Brand new games.
As for mark-up, I just use a same time old strategy that has worked for me, and that is a Minimum of 100% mark-up or Minimum $100 net profit on higher ticket items.
Like anything else, you need the best suppliers (not "middlemen") and of couse, a lot of research.
I notice that you and a lot of others sell jewelry, I've considered it but although I have the business knowledge, even contacts, I don't really know anything about Gemology (is that the right word?).
Bottom line tho is that most of my games are anywhere from 300-1000% mark-up (Gross). I don't know how anyone could survive with less to be onest-injin.
-Jeff