Ask HN: How to pay your users?

I'm in the brainstorming part of trying to start a new app. Similar to Amazon's affiliate program or the way 99 designs holds design contests, I'd like to be able to compensate the site's users for their input.

For those getting compensation, I'm thinking the average amount would be less than $100.

What are some hurdles to paying your users? What legal obstacles should I be aware of? Is this a solved problem (i.e. should I go with PayPal? Stripe?)