Sometimes, I can be pretty picky. I like to order the same thing at restaurants and often choose which restaurant to go to based on which main course I'm most hungry for that night. I tend to find things I like and stick with them. There might be something better out there, but I'm OK with what I know I like.
The same is true in Warcraft. There are certain vendors I always visit for certain tasks. There's Ginny Longberry in Ironforge for all my reagent needs. She was the first NPC I found in Ironforge selling the reagents I needed, so I've continued to patronize her. She's conveniently located near the mage portal destination, so my mage continues visiting her to this day.
Until the Shattering, Auctioneer Jaxon was my go-to auctioneer. She always seemed the most friendly, plus you could access her through the wall while next to the mailbox. After moving to the Dwarven Quarter, I've become attached to Lauffer, Billibub Cogspinner and Kaita Deepforge for my auctioning, vendoring and repairing needs. Leslie Wainright handles my bank account and Jennea Cannon provides my training.
Which NPCs are you loyal to? Or do you just do what's convenient, not caring who gets your business?