It varies depending on country, where you are in the US, time in foreign customs , time of year, and the way you send the parcel
. If you're going for first class then from a UK standpoint I've had things arrive are fast as 3 days to a week (from the east coast) or take as long as 6-8 weeks. I've had stuff held up in customs a week before i got sent notice of the import tax needed to be paid before they would deliver the parcel too.
The tax is something else your potential buyer needs to consider as this can get very high - especially if your collection is valuable.
What country is your buyer in? it would help to know as we have a lot of other Euro members as well as us UKers who might be able to give you an approx. time for their country as well