I am a business attorney and not a tax advisor. I have, however, been in the position of researching and advising many clients on this point. So, with the caveat that you should not rely on my tax advice, but rather consult at tax professional, I can offer you the following:
Electronic payment systems and software is a fairly developed space. There are many options and you will want to look at several to see what fits your needs best in terms of minimums, fees, payment type (MC, Visa, wire transfers, etc.) Popular software as a service providers are www.authorize.net, www.paypal.com, www.paysimple.com.
Also, take a look at the TechCrunch article about the online payment wars among service providers at http://techcrunch.com/2009/07/23/the-online-payment-wars-continue-paypal-officially-announces-flexible-api/.
Your primary concern is you own obligations to the IRS if you are US-based. You are required to maintain meticulous records of payments to contractors including verification of personal information for US citizens. You will require a 1099 completed by the contractor.
For foreign independent contractors, called nonresident aliens:
Services performed in the US - You need to have them fill out a Form W-8BEN and you will have to withhold tax liability (for work derived from US-source income) unless they provide you with an exemption in IRS Form 8233 that is based on tax treaties the US has with various countries. Otherwise, the withholding is 30% normally. MAKE SURE YOU CONTACT A TAX EXPERT ON THIS POINT.
Services NOT performed in the US - there are no reporting or withholding requirements other than your usual tax disclosure in the US.
The only other pitfall that comes to mind is the export controls on products, goods or services that could be classified as having 'dual use'. That might include any outsourcing activity to a restricted nation of software, information, data or other information that "could" result in the development of military weapons (check the Department of State website for details).
I hope this helps and isn't more confusing than having no information at all!