I used to run a website called "Perlhelp.com" and would do that.
I got some work from it, but I also lost my ass a few times because I didn't charge up front and there was one particular guy who'd tell me after I sent them a solution "I've already got this done".
Since I couldn't review his code I couldn't know if he used my code, but I do suspect he did screw me at least a few times and I had to decline his requests for help after a few too many times of that.
There was also getting up to speed on the real cause of the issue they were having. That can require some time regardless of your proficiency with a particular language.
I think concept is good, but I'm not sure a fixed rate to "Fix this issue" can work out well for both parties all the time.