Draft and sign an i/o(insertion order) which is the legally binding document between the advertiser and the traffic providing party. The i/o will state your payment terms, the cpa/cps, any stipulations like exclusive rights, what type of traffic you are sending, and anything else that needs to be covered to protect the deal. If you run sales without getting one of these together and you get burned there are little options for recourse.
Some sort of decent tracking is needed, advertisers do not always have their own. They may judge based on what type of tracking you are going to use because there are always discrepancies between their actual sales and your reports. Place pixels on the landing page and the order confirmation page of the offer.
If you don't understand or aren't comfortable with these steps I would not bother going direct.