I ran a mailorder computer games store with two shops for nearly 5 years and it's very hard work I can tell you.
Games are all bought from distributors and you get various discounts off the rrp. We got anything from 25-60% off rrp.
We mainly sold Amiga and PC games but dabbled a bit in console stuff.
We used 1 main distributor (Centresoft) and a few other smaller ones for the odd weird game that noone would sell.
There are a LOT of overheads to consider as well. Packing material, credit card stuff, postage, staff wages, phone bills, e.t.c.
What killed it for us was the sheer amount of returns you have to deal with. Back in the days of Amiga and Pc games coming on multiple floppy disks and where directx didn't exist, games just didn't work on loads of peoples computers and you are obliged to refund or replace the game. Trying to get distributors to replace games is also a nightmare.
Thankfully with directx, cd's and better pc's, things are a lot simpler now.
P.S. We were doing a turnover of well over a million quid a year, which you would think would make you rich. How wrong you would be!
Also, if you have a mail order company you are obliged to do lots of advertising and this is the biggest expense of them all. It will cost you thousands of pounds every month.
Good luck!
Boo!