Oh yeah, before buying, do this.
1. Call them. If you can get a real person on the phone within 10 minutes, then the company is looking good. Continue to step 2.
2. E-mail them one question with a general product inquiry, e.g. "do you carry this product?", and another one with a particular shopping question, e.g. "you advertize this on your site, is it in STOCK and when would I receive it if I ordered it today?". If you get anwers to both questions within 2 days, the company is looking good. You can order 🙂
NewEgg.com, mWave.com and Buy.com never failed the test, and all are pretty good companies. Some other companies, however, took forever to get back to me, and that's the kind of support they provide. They suck.