The marketeers are not made up of anything.
It is just an account with several people having access to it.
An TSA who I probably should not name takes care of the transfers and a bunch of people who all happen to be Marketsquad or Ex-Marketsquad take care of the account's inventory .
Now to be actually useful:
Here is the requested link
http://forum.toribash.com/forumdisplay.php?f=489
They can sell you high qi stuff you normally would not be able to receive for loads of TC.
They do not accept real money or the like as their goal is to prevent an inflation of the TC.
If you mean marketeers in the general sense: They are people who try to earn money by buying items low and selling them high.
Everyone can be a marketeer. All you need is common sense.