I've done it, but you have to be VERY careful. Keep in mind that when you sell (on craigslist or ebay) you're selling INTO the dented, scratched, used, clearance, marked down economy. It's a little harder to turn a profit there.
Example: I bought a Fender MIM Deluxe Jazz V for $125. These were $675 on musiciansfriend.com at the time. It was lightly used - nothing a little attention and setup didn't fix. It was in "Guitar Center Showroom" condition when I sold it (i.e., some blemishes, but functionally perfect). This bass is now sold for $750 online.
When it came time to sell it on craigslist, how do you think it went?
I was asking merely $350, and TWO people came to my apartment, held it, played it, and PASSED before I sold it to a third guy.
Put a MIM Fender Deluxe Jazz Z on TB for $350 and see what people say about it - it was a fantastic price for a lightly used instrument, and it was still difficult to move.
My point - if you see a deal and think "Ooh, I might flip that for $100!" you must STAY AWAY. You have to say, "Ooh, I could flip that and make $300 or more!" If that works, you MIGHT have a good deal on your hands.
Stuff I never sold:
Cort Curbow fretless for $50 (traded into a Peavey T-40)
Sunn Concert Lead amp for $75
Peavey Patriot MIA bass for $50
Fender MIM Jazz for $100
Some Peavey PA amp for $50
I ONLY get involved if I have a chance to buy a ROCK BOTTOM prices.
I also got stuck with a Schecter bass guitar and a Crate combo amp (two separate craigslist deals) that I could not sell at any reasonable price. These both ended up at GC. I ended up breaking even on these, but ONLY because I acquired them at an insanely low price.