The
Mr.Rebates one is best and it has nothing to do with loyalty. Just login through their site and buy whatever, whenever you want. Aside from the slight hassle of logging in their their site there is absolutely no catch or requirement. Heck if you do it properly you can even stack it with airmiles as well (if you want more info pm me about this).
The ebay bucks one isnt loyalty either because it covers everything on ebay.
Basically at the end of the day its all the seller's fees and stuff that funds these two programs.
On any discount or rebate program, they take money from everyone and give it to a limited set of people. If you make sure you are in the limited set of people you will pay lower prices. This is a general principal, not just limited to ebay.
As a personal note Ive already racked up $17+$5 Signup bonus in cashback within the past two months.