TIL there's a speech perk tree. Thanks.
NP. Perk #3 up the left side is a huge help (sell anything to anyone). The one above that (increase vendor gold by 500 permanently w/ investment) helps some as well, but it's a necessity if you're trying to get the top perk (all merchants +1000g). For the most part I think #3 is the only "must have" perk. The investment one is helpful but not huge, and the master perk is just too many perk points to spend on speech, IMO. There's a "sell stolen goods" perk along the way, but I think it comes too late in the game to matter unless you're RPing a professional thief.