At least 80% of my stuff is from eBay. It takes a few weeks to get things, but you can't beat the price. Most things come in very good condition. I ordered some pots onetime that looked used, but besides that everything was(or at least looked) new. I've bought things from Sparkfun, Jameco, and Seeed Studio, too. Sparkfun has great customer service and fast shipping. They also have some pretty cool/interesting parts, plus most of their designs are open source and they support the open source community. They're not always the cheapest, and they don't have a huge selection, but they do give back to the community. Jameco has a very large selection, fast shipping, and good prices. After my first order, they sent me an email for free shipping on my next one. I only bought from Seeed Studio because something that I would've bought at Sparkfun was out of stock. It came slowly, but only because I bought it during Chinese New Year. They also have PCB and laser cutting services, which is pretty cool.
SO:
Sparkfun:
- Customer friendly - OK prices(mostly) - Fast shipping - OK selection
Jameco:
- Good prices - Fast shipping - Very large selection
Seeed Studio:
- PCB and laser cutting services - Good prices - Slow shipping(they're in China) - OK-ish selection
eBay:
- Super low prices - Slooooooooow shipping - HUGE selection(you can find almost ANYTHING)