You have vendors in almost every star base type place. Cant remember if vulcan has vendors. You have SB-1, Sierra SB-39 in Alpha Centari, Andoria, the vendor ships in Sirius and Regulas (you also have another weapons vendor ship thats pretty hard to find... Believe its Captain Mudd or something like that.), DS9, the neutral zone at K-7(its the pvp gear vendor place), and the fleet group in the delta quadrant. Klingons have Ganalda and Qo'nos (havent leveled a klingon past lt Commander so I dont know if there are more than those two places).
And you can buy/sell junk using your in ship replicator.
someone did post info about the vendors at SB1 who trade in exploration badges, also i saw a site linked that had a map of sb1 and a list of the main NPCs including vendors. i did a list of DS9 and am doing one on SB39... not seen one on andoria or K7 yet.