i've traded unusuals for a while and here's what you should look for when you're starting out.
1st gens -
p.confetti / g.confetti
circling heart ; this is usually more expensive as a mid tier unusual but sells well
plasmas ; either good low tier or low mid tier, really depends on the hat , class etc
g.energy ; it's the cheaper one and you can find some good deals, just down impulse buy when you see a g.energy going on quicksell , evaluate the hat and class it's one and how well other hats with g.energy which are similar are selling.
2nd gens (mostly low tier garbo)
blizzardy storm ; if you have a choice go for a stormy storm , unless it has a well fitting theme etc
stormy storm ; it's decent , usually bought often
3rd gen -
disco beat down ; while it looks great it's not bought that well but you can always make profit on this effect (probably)
miami nights ; a classic and a good effect , bought well.
i'm not even gonna bother with taunts and you said cheap effects so i wont bother with haloween effects . (eerie o.b and flaming lantern are both usually quite cheap but are both kinda bad in most cases)
i've probably missed some but this is what i can add 8 - )