Like everyone said, "buy low, sell high", buy trading cards,
scamming (oh, HappySticks...), and gambling for your rewards are ways to make profits.
I would recommend "crafting" your profits. What I mean by that is take advantage of Chemistry Sets and Killstreak Fabricators.
Chemistry Sets (Strangifier):
If you got a chemistry set for a strangifier, check for the recipes and their most expensive craft recipe, which is usually a strange weapon.
If the value of the most valuable recipe (and also the sum of all the recipes) is greater than the outcome value of the strangifier, then you're better off selling it to some else (i.e. Strange Kritzkrieg as a recipe for a 8 ref strangifier).
But if the value of the most valuable recipe plus the sum of the recipes is less than the outcome value of the strangifier, then work to buy and create the strangifier and sell it for a profit. (i.e. the sum value of my recipe is 1.66 ref for a 8 ref strangifier. Made it, sold, and gained 6.33 ref profit.)
Chemistry Sets (Collector's)
If you obtain a collector's (whatever item it is) chemistry sets, you can do one of two things, make it or sell the set. But if you plan to purchase one, make sure the price is reasonably low so that the amount you spend don't go overboard with the value of the collector's plus the recipe value. (i.e. Spend 3 keys on a collector's, spend ~2 keys on recipes, outcome collector's value is 4 keys. Not worth it!)
If the required item is a weapon that can be used for scrapbacking, ex. Flare Gun, then you can easily obtain them by the method of scrapbanking (I recommend using a scrapbanking website for this) and spend only ~11.11 ref and receive profit by selling it by it's asked price (i.e. Collector's Flare Gun for 4 keys) or just sell above ~11.11 ref. You can buy the weapons for a scrap each if you wish, but the scrapbanking method is more economical.
If the required item is a hat or something else, check how much metal you need to make it (200 x value of item= recipe value). If you can make it, go for it! If not, then sell it.
Killstreak Fabricators
If you have a pro/spec. KS Kit Fabricator, the best way to get the ingredients is by buying them off the steam community market. Parts will go for $0.03-0.08 each depending if its battle worn or reinforced, and Pristine will go for $0.15-0.20. Make sure you also get a cheap killstreak kit that is used on a cheap weapon, such as the Gloves of Running Urgently Kit.
Total amount for (only) the recipe by using this method is: ~$1.80 for a spec. KS kit (in ref= ~5.66 ref) & ~$5.40 (in ref= ~16.66 ref) for a pro. KS kit, but exact cost varies.
Since there's no set prices for the killstreak weapons, you can set up a price that you like. I recommend selling the Spec. KS ones for >1.5-3 keys & the Pro. KS ones for >4 keys. You'll get a profit above >0.5-however much keys.
Overall, it's only a recommendation. You don't have to do this if you don't like it. If you do, I hope I helped you a lot and just wanted to show a little variety when it comes to trading and profits. And of course, when it comes to making profit, hard-work and patience is involved but it works in the end!
P.S. I've tried to make it short as possible, so I do apologized if I've bored you to death with this. But if you read all this, here>>>>>>>>>>
EDIT
The planning of purchasing a Killstreak Fabricator still applies on making sure you don't overpay for getting the recipe, such as the Collector's Chemistry Set. Most Specialized are $0.03 and above on the market and many Professional are ~<$2 and above, so choose your Fabricator according to your comfortable buying price.