What should I loot?
Well, for a start, jewellery. Rings, necklaces, whatever you can find. Of course, chances are that the bodies you loot might not have any, but if you see it, pinch it. Make sure you can appraise it to see if it's worth anything.
The main thing you should bear in mind when stealing things from corpses is the portability of your loot. Weapons and armour are all valuable, but they can all weigh quite a lot, and a person trying to lug eight sets of scale mail around is not likely to get far. Even if people don't ask him awkward questions, chances are he's not strong enough to move that all by himself.
Next, is your loot traceable? It may well be. Swords can have armourer's marks stamped on the ricasso; shields are often painted in regimental or heraldic colours. If you're trying to sell the arms and armour of a vanquished regiment in a city to which that regiment belonged, chances are you'll get run out of town at least, or hanged at worst. Think you'll find a buyer for Sir Guthwald's one-of-a-kind silver-and-emerald ring with distinctive etchings around the inside of the band that read To my beloved Sir Guthwald from your loving wide Lady Trina
? Not among any reputable jeweller, you won't. Be sure you know you can sell your loot at minimum risk to yourself!
Another thing to ponder is your potential market. It's one thing to find loot but another to find someone willing to buy. Offloading stuff onto fences may seem a perfectly reasonable way to go, but if you're selling stuff that may still have the grime of battle on it (or if you've had to take drastic measures, may still have fingers attached) then don't expect to get a good price. Stolen property is worth less than the seller might expect.
Now, looking back to the title, a few choice treasures may be had, straight from the corpse's mouth. Gold teeth have a small resale value. That said, if sugar is readily available in your campaign, then, logically, so is tooth decay. A bit of work with a dagger can get you upwards of twenty teeth. Who knows? Teeth might be saleable at 1 sp per dozen to the right man. Think of the demand for sets of dentures. Recommended price for sets of false teeth: 3 sp each.
Articles of clothing can come in handy too. There is always going to be a demand for cheap clothes, and, as much of a cliché as it is, a body-looter can always make off with a corpse's boots if it has nothing else of value on it. Do bear in mind what I said about portability of goods, however.
As a rough guide, remember that the resale value of most items is roughly half the amount of money a first-time buyer would have to pay for it.
When should I loot?
Good question. If you're looting the remains of a large-scale battle, you should have kept an eye on the proceedings. Wait until it's over, and when you're sure that no-one's watching you, make your way to the scene.
Of course, it goes without saying that you shouldn't loot corpses in the middle of a fight. Unless you're bloody stupid, that is.