if you learn how to use ebay efficiently, it can be a godsend.
some examples: (all prices are in Canadian Dollars unless stated otherwise)
Fireblast (visions common) retail $0.25 at local card store
i saw 4 go for $7.50 US the other day. dont ask why, they did though. retarded.
Stroke of Genius (urza's saga rare - Foil) retail $125 at local card store
purchased one awhile back for $30, traded it for 2 morphlings and some other blue rares. i've also seen this one go between $25-$45 on ebay.
phyrexian negator (urza's destiny - rare) retail $7-10
i picked up 4 on ebay for $16. not bad.
foil yawgmoth's bargain (urza's destiny?? - rare - foil) retail $35+
i picked up one on ebay for $8. cant beat that.
yawgmoth's will (urza's something maybe, i cant think too clearly right now

- rare & best black card) retail $6-8
i picked up 3 for $10 on ebay.
tek (invasion dragon, sucks - rare)
bought 4 for $2. why? because at the time they traded for about 6. thats $22 profit.
as you can see, the prices fluctuate and you have to know how to use ebay efficiently to maximize profits and minimize losses. with mtg, its about supply and demand, as well as little kids. for example, a friend of mine traded four $6 rare cards in some stupid wwf wrestling CCG for 4 3rd edition sinkholes. he then sold the sinkholes to a lam for $70. meanwhile, the 4 cards he traded were purchased for $2. he is a smart kid, and did a good transaction, profitting $68.
basically what you need to know about buying/selling mtg (trust me i know, i do it a lot) is Supply, Demand, Buyers, Anti, and Little Kids. if you can scam cards from little kids that don't want them, perfect example was sinkholes for WWF RAW cards, then do it. they are happy, you are happier. once you are good enough to play anti, play anti and win cards. make sure you know what you're doing here though, it takes a lot of time to learn how to play effectively and how to make your opponent feel like he's winning/going to win, then take the cards you really want when you need to, kinda like poker.
if all else fails, steal...er...trade them. haha. dont steal, unless you hate jesus in which case steal away!