Are you gambler? There have been many people who have built their multi-time using the buy/fly/sell model. For some, it worked out great. For others? Not so much.
First off for the business model to work, you've got to buy/fly/sell within one year if at all possible. The most ideal scenerio is to buy the plane with a fresh annual, get your 100 or 200 hours out of it in the first couple of months and get it sold before you have to put another annual on it. Also ideal is to buy the plane outright instead of getting a loan so you can forego the need to insurance on it. Just don't bend it while you own it. If you can do it this way, and you manage to resell the plane at 98% of what you paid for it or better, then you can build multi time very economically.
But like I said, that's a big gamble. What if you end up toasting an engine a month after you buy it? What if the gear won't come down one day and you have to belly it onto a runway? What if the market tanks and you can't sell it within a year, and it needs an annual? Then renting suddenly becomes the cheaper option in all those cases. The thing that makes renting attractive for time building for so many people is that you know going in exactly what its going to cost you. With the buy/fly/sell model, you can only guess at what it will cost you and you need a whole bunch of ducks to line up perfectly in order for that number to become a reality. Renting is more expensive, but it's also the safer gamble.