you can "join" the thieve guild for free

[tHe_silent_H]

If you talk to the fence and enter the trade screen(don't have to buy anything) you gain access to the surgeon and the training gear.(you shouldn't be able to trade with the trader in the first place. talking to the boss or the surgeon prevents this.
"Joining" this way prevents you from talking to the thief boss, so you never get the relation boost.