I think a lot of people have the right idea on here. Food, ammo, and pretty much whatever someone has can and will be traded. Women will sell themselves, men will sell whatever goods that dont have longterm value. Food is a something I don't think will be shopped around very much. Food will be harder to come by for everyone, those who have it will think long and hard about trading it off.
One thing that I think will make great trade material is moonshine. Easily made, and has more uses that I can count. Fuel, cleaner, antiseptic, anesthetic, and recreational use. The fact that it can be burned in a vehicle will make it very desirable.
Precious metals will always hold some amount of value, but I guarantee that it won't be nearly what is currently. For people that have someone secured pieces of land from walkers seed will be a tradable resource.
I wouldnt be surprised if water to some extent doesn't gain trade value. Especially for those foulish enough to try to stick it out in major cities.
If your smart, get out of the cities and fortify some space of land as best you possibly can. Stock, up and plan for the worst possible.
gasoline, ammo, bottled water, medicine, weapons, liquor and portable food.