I flip guitars. You just have to know your market really well and focus on a particular style of guitar or manufacturer. Also, sometimes you may have to sit on a guitar for a while before it sells, but if you stick to your guns, it'll sell eventually. In my most recent sale, I had to sit on an axe for the better part of half a year. But generally, sales can be as fast as a week or two.
My area of expertise is high end, 80's style super strats. Just know your market really, really, really well. Also, with higher end merch, there's more room for a profit, price and a little built in haggle room.
-Just a tip