I had the same idea you have, and I started doing exactly that about three years ago. Here are some truths I found, for what it's worth:
1) It's tough to sell dirt bikes for book value; you need to buy them way below wholesale to make any money.
2) It's often expensive to fix hidden defects, and as often as not you lose money on the bikes. Plus, while you have the bike, you will want to ride it a little. Who wouldn't? If something breaks, you'll have to fix it before you sell it. I bought a year-old YZ 125 for a thousand bucks below wholesale and thought I was going to make a pile. It seemed like a great bike: glossy and crisp. I took it riding a couple of times, and the top end gave out, blew combustion down into the lower end and completely ruined the engine. Needed a new piston, rings, cylinder, crank rebuild, bearings, seals, etc. Like Super Hunky says, there is nothing so worn out as a year-old 125 that's been ridden hard and maintained poorly. I lost my shirt on that bike when I finally got it sold. This will happen to you eventually, if you ride the bikes you buy.
3) It's expensive to advertise, and it takes up a lot of your time showing bikes.
4) You will be inviting all kinds of people into your garage, possibly into your house, to get a good look at what you own. There are lots of nice people interested in motorcycles. Some of the people who come, however, will be criminals looking to rip you off. I have had such crooks in my house, as I was later able to determine. By the way, these thieves often identify themselves as policemen, and may flash some kind of badge quickly. Don't trust anybody, don't take a check, and try not to show them any more of your household goods and garage stuff than necessary. I bring the motorcycle out into the driveway and close the garage door. They don't need to see the other three bikes, the drum set, the guitars and amps, the tools, etc.
5) Auctions can be good places to buy bikes cheap, if you are disciplined, and don't get caught up in the bidding and go too far.
Eventually, I found that I was unwilling to put up with the inconvenience, the hassles, and the parade of people trooping through my place. I have given up trying too hard to make money selling dirt bikes. I now use the knowledge I gained to buy bikes for myself and my family at very reasonable prices. I can usually buy a bike, ride it a year, and sell it for what I paid. That way, I don't lose too much money on depreciation of my machines. Sometimes I even make enough to defray some of the costs of upkeep. If I see a great bargain, from time to time I will still take a crack at making a few bucks. But I don't expect to get rich doing it.