It's fine if you communicate it up front. I'd make sure this was somewhere in a nice big font on sign-up: "You're using a free service, and we need to make mo

It's fine if you communicate it up front. I'd make sure this was somewhere in a nice big font on sign-up: "You're using a free service, and we need to make money, so we modify certain links to e-commerce sites. When people click on those links and then buy something, we get a small percentage. This doesn't affect the content of the linked sites at all - the experience will be identical. It does mean you'll be unable to make money on your own by linking to e-commerce sites and using an affiliate code. Sorry - but this is a free service." In fact, removing affiliate links almost certainly has a beneficial effect - your service is less likely to be seen as a tool for direct marketing lowlifes. A lot of things that seem shady when done on the sly and hidden from users are fine when explained up front and in clear and simple English.