I'm integrating my website with a merchant account. What would be the best site to outsource this?
I preferred elance.com in the past, and now they merged with oDesk.
Yeah, it's tough to know who is legit or not and what is a fair price. If I go cheap, then I get shoddy work and problems.
Onlinejobs.ph or Freelancer.com. Onlinejobs.ph is geared to those seeking full time employees. Freelancer.com is more project/project basis, but I got lucky and found my current full time coders there.
Last time i tried to save some money outsourcing i had to delete an entire website+database because the coder (who I paid to buy and install a premium wordpress theme) downloaded it from torrents websites.
That wordpress website and database was compromised due to a backdoor in the WP theme, and in no time it was sending thousands of spam emails every hour. I risked my entire hosting account for that reason.
How difficult would it be to integrate my site with a merchant account?
I've used people on Odesk for these kind of tasks and it has worked out well.
I would stay away ( my experience ) form indian / chinese contractors.
I've had the best luck with eastern europeans and US coders.
Why stay away from Indian? I've had only Indians apply...lol
As I said this shouldn't be a generalization, just what my experience has been so far.
Cmdeal gave a good explanation why in most cases indians on Odesk won't provide amazing quality.
Of course, there are always exceptions to the rule 
Well for Odesk, if someone messes up or totally screws you over..I assume you can get a refund, right?
odesk and hire eastern europeans... i hardly ever have luck with indians or phillipinos there ever. (i did meet and still use services of a great guy from india that works for oracle, the guy is crazy when it comes to excel)
The owner of easyoutsource and onlinejobs.ph is an STMer you can hit him up for tips. John johnas.

bummer 
If you want to take a very different route and get someone really good - but probably not cheap - my usual recommendation would be the Hacker News monthly "Looking for freelance work" thread. You can find it on the "Ask HN" section at http://news.ycombinator.com .
Seriously world-class coders advertise there - but seriously world-class companies also hire from there, so you won't get people at oDesk prices.