Does your friends account have a house? If so, check on the porch at the side of the house and see if there is a consignment merchant there. If so, that is how you list stuff for sale. You put items on the consignment merch and set what price you want to sell it. Even if the house is owned by a different character on that account, all characters on that account can sell on it. To find out if a character owns a house, check the stats page of character info to see if a louselot is listed.
If there is no house on ANY of the characters on that server, most random junk drops can be sold to any merchant. If its nice gear, it is a lot harder to sell it and get anything out of it. There is a channel set up for buying and selling items. /trade will help ya locate buyers of rare, much wanted items.
As far as where to get nice gear, that depends on your realm/class/level.