it is against eBay policy. They can ban you, give you a strike (warning...) the savings you have is pasted mostly onto the buyer, not to the seller. Ex. If you are selling an item for $100, and a buyer asks you to do it off eBay for $90, its not worthwhile, as the $10 savings is your fees. Either way you would be walking out with $90!
The only thing you get out oof it is the loss of seller protection. (as stated earlier)
If we were talking about a constant buyer that would want 5 iPhones a week, and is asking for a $25 discount each- it would be a differnt question.