It can be done. I've flipped a few items.
Couple of points:
You've got to stay glued to gumtree and act quick when you see a bargain. There are plenty of people trying to find bargains too. It's kind of a full time job, but has the advantage of being a job you can create for yourself.
You've got to flip things quick to make money, unless you have deep pockets and a lot of storage space. You will often get a better selling price if you bide your time, but unless you've got a decent bankroll and some where to keep the item, it makes more sense to sell for a smaller profit.
Don't forget to keep things like transport costs in mind when thinking of your selling price. How much petrol did you use if you picked up an item from the other side of town.