I've been listening to a lot of talk recently amongst survivalists, preppers, and the like, on what to stock up on, when preparing, for purposes of trade afterward. Food, water, ammunition, many things have been discussed. Popular are high silver coinage, or ones with a recognised gold content.
What you would need obviously depends on what future you expect, how many people are around - and what mood they are in...
I would think, though, that the best thing to trade is something you don't lose when trading, and something that is much more valuable with people wanting you to stay alive rather than killing you...
Traditionally, that means skills... Medical ability, trade skills... things that people can't take off you, things that someone would lose if they kill you - effectively making you, to use the words of economists, a non-fungible. Anything that means that you could not be easily replaced.
A strong pair of hands is not enough - there would be enough people willing to dig furrows, cart water, cut firewood, or similar, PSHTF - office workers or business people who can work well within a post-industrial civilisation, but have few skills otherwise. Hunting, as well as the requisite skills at dressing, are fine when there is enough game to make it worthwhile - and you have enough skill to support yourself.
This is assuming your plan isn't to take what you want by force...
The next best, possibly, would be to get things of little value, or that you can be paid to remove, processing, and then selling back. Case in point - urine. Even into the 1850s, urine was a valuable product - you can ferment it into ammonia, process it for phosphorus... Apparently, it also makes a good fertiliser when mixed with wood ash, possibly through a reaction between urea and potassium carbonate. You could do a lot, with some chemistry knowledge, although the smell is a problem. And it's not that long ago that night-soil men (as they were called) went around, generally improving life and hygiene. Getting rid of that, finding some use for it may be another issue - methane, perhaps.
Value is in the eye of the beholder... I'm trying to learn a little bit about native food plants - or even plants that don't look like food, especially to the uninitiated - and known medicine plants. Tea trees (as in the oil) are good - but steam distillation extracts the useful components. Camphor plant is a weed, but the chemical camphor has use.
The final suggestion that I would have is something of moderate value, which you can make even more valuable. Fruit is good, but you can only store so much before it rots (or have only a certain amount of ability to preserve), but fermentation (and even distillation) gives you something much more easily stored and traded...