I searched, and couldn't find anything. So sorry is this is a repost.
You need to be druid/paladin/death knight/shaman/warrior/priest. Or have one of them in your group.
Lets say it starts like this; *buyer* is in Undercity, *seller* is in Thunder Bluff.
Invite the *seller*/*buyer* depending on whichever you are. Queue for regular randoms, or regular specifics, and if you are druid/paladin/death knight/shaman/warrior/priest, queue as tank or heals. If the other person is druid/paladin/death knight/shaman/warrior/priest, tell them to queue as tank or heals. (Or both works, probably even better.)
You get a group very instantaneously on most realms. Once in the dungeon, trade with the other person, and leave the group. Voila.
Tip: You may want to do a specific dungeon, so that way when you enter you don't get the random LFG debuff. This also would be why you don't queue for heroics. Once you leave, you get the debuff, (although there are ways to avoid that.)
Don't hate if you don't like the guide. kthx.