and what do you mean by "selling epics"? As in running through high level instances for money? Or do you actually mean epic drops, because you can just go to the AH for that.
Considering he's talking about end game guilds, I'd assume he means guilds selling loot out of MC/BWL.
It's fairly simple how it works. You get a spot with them on a raid and either pay X gold for all of your class loot from the run or Y gold for each epic you buy.
I mean guilds that have end game instances on farm status and sell epics, you can buy them directly from a list or get a spot in the raid and buy the class drops, as simply as that.
If you want to transfer to the realm, Bloodscalp, there's PuG MC and BWL that you could attend. If you really want to buy the items, there are guilds selling loot, too.