What percentage do you pay for used modern retail stuff?  

Condition
Depends on the condition. Pay what is reasonable to you. Personally, if something is new old stock, like-new, or mint condition, I would pay up to 2/3 of the retail.
niceguy would probably pay 10%.

maybe
woodywood
My typical number is 15%. I require very little and it makes it easy for me to say no to higher prices. Never be afraid to offer less for an item, you can always raise your offer. Good luck and woodywood thanks for thinking of me.

I base my percentage
entirely on how much loose change I think there might be in the cracks. Works every time.