1) I usually don't concern myself with the return policy. This is for two reasons. One, I usually am buying something that I am familiar with and don't expect a return to be necessary. Two, I am usually only buying from reputable, well established companies. If by chance I am thinking of buying from a company I've never heard of, I will look at the return policy, but I don't hold too much stock in it; if they aren't a reputable company, why would they honor their return policy?
2) A fair return window for online purchases is sometimes difficult. I don't think you can say "this many days after purchase". You need to use a delivery carrier's tracking system to say "this many days after delivery". I think it would need to be at least 15 days after delivery, preferably 30. I often have bought an item and haven't had time during those first 15 days to set it up.
3) Another difficult question. There are varying degrees of not in "immediately saleable condition". Then there's the question as to why it is not in "immediately saleable condition"? Was it a shipping issue/poor packaging/etc... Was your packaging designed so that the only possible way to open it is destroy the package? If it is my fault that the item is not in "immediately saleable condition", I would be willing to pay a 10% fee. I would be dissapointed at having to pay 15%. And I would most likely skip the retailer altogether if I have to pay 20% or more.