1. Return policies are very important. Too often, problems arise after packages have been delivered and opened, and sometimes, they happen well after that. If I can't return or exchange an item, it's a pretty long shot that I'll ever give my business to that company, no matter how good their prices are.
2. Since cables (particularly digital ones) either work or they don't, then 30 days is fair. [Although to be honest, I answered 60 days without even thinking about it.]
3. 10%. We customers are unfairly taken advantage of enough as it is, so 0% would be nice, but unrealistic. 10% seems fair to me.
cheers,
supervij