You have to be careful of course its not just $5 though, I mean you can pay $5 but you can also pay for gig extras if the seller has gig extras.
It's just about using common sense and looking at the reviews as well as thinking about what the real cost would be to actually do the work.
As I say there are some good services to be had you've just got to look for them.
I have had good and bad experiences, you can get some services on them that you'd pay 3-10 times more on other sites. That is no joke either haha.
There are some things I would avoid but if you are on a budget then for the sake of spending some time looking through reviews or just testing with a small amount if it does not go as you wished then at least you have not wasted all that much.