Normally i do the "Buy cheap and sell a little more expensive" thing, but it doesn't work for me.
Also i've played MvM Mann Up 4 times just hoping to get something profitable.
Just bought for 2 tours and got enough for the other 2 tours by trading with all i got from that  (I've sold all my killstreak kits to bots cuz nobody wanted it), obviously i haven't got anything profitable in all those tours.
I've seen a lot of videos of making profit, but i can't get something decent, i just can get scraps from the strange trading (If i have luck) and it's hard for me to find some profitable hats (If i find one, i also get a scrap or if i'm lucky, 2 scraps).
Literally i can only make 1 key and a half in one month or more while other people make more in that amount of time.