Start by educating yourself, on how to spot a good bike, and what is the market value of such bikes. Then pounce when you see a deal. Deals don't last long enough to ask "hey is this one a good deal?" While you are asking, if its a good deal, someone else is grabbing it. And of course, you need the tools/time/aptitude/knowledge to fix and repair the bikes yourself (paying to have them repaired will reduce your "profit" to a "loss".)

You really have the cart ahead of the horse. Study now, buy later. To make decent money on flips, you need to be able to decide in about 15 seconds whether a bike is a deal or not. While others are pondering, you will be buying (or walking away....)

The best deals tend to not be on C/L or ebay, if they are, they don't last long. Better deals are found at thrift stores, garage sales, word of mouth, neighbors, and so on. Even then, the deals do not last. Around here, great garage sale deals last an hour or less. Great thrift store deals last maybe a minute, no joke. With the economy as bad as it is right now, there are more people than ever before looking to flip stuff. Went to a garage sale this morning, got there an hour early, they had three bikes, all duds, and scoopers were grabbing them as I walked in.......

I just flipped a clarinet this morning. I look for deals, on pretty much anything. I don't know anything about clarinets, but I can recognize a deal.