[charleshaanel]

"you can't split test their landing page" - Sure you can split test. Simply drive the traffic to your own list first and create your own landing page (the affiliate link on the lander is your own).

"You lose a lot of traffic to people that directly type in the url." This is why you buy your own domain and only pay for traffic that goes to your domain. If needed (ex. the merchant starts giving you drama, the offer doesn't convert) - you replace it with a redirect.

You use other people's offers to test traffic sources, email sequences, landers. Then once you get it right you build your own products. People have been doing it for years.