Lead, pure and simple.
If you want it and I have it, lead will prevent you from taking it from me. If you have it and I want it, lead will ensure that it will transfer from your possession to mine.
I'd imagine that the best barter items would be things that we take for granted every day. Medications like pain meds and anti-biotics among other medical supplies and know how would be invaluable. If you were to save someone's life, or stave off a serious infection, they'd feel indebted to you. This would possibly lead to a long list of favors that you could draw upon.
Candy, coffee and shelf stable food would be good trading items. Gasoline would probably be a big thing, as would potable and palatable water. Water purification items would be a hot item. Sterno, heat tabs or other cheap and easy heat sources would be good to keep on hand.
Leather and fur tanning supplies would be handy. You could probably get something for a good spank book and a sample sized packet of lotion. Personal hygiene items might be hot too.
Also, many people already appear to be in dire need of a spell checker and those aren't even in short supply.