I was originally going to go with the Greyhawk Money System, but it hurt my head, and I am not certain you guys want to get that in-depth. Just easier go move to the official D&D Coinage exchange.

Copper 1 1/10 1/50 1/100 1/1,000
Silver 10 1 1/5 1/10 1/100
Electrum 50 5 1 1/2 1/20
Gold 100 10 2 1 1/10
Platinum 1,000 100 50 10 1

Selling Loot

Opportunities abound in D&D to find treasure, equipment, weapons, armour, and more in the dungeons you explore. Normally, you can offload your treasures and trinkets when you return to a town or other settlement, where you can find buyers and merchants interested in your loot.

Gems, Jewellery, and Art Objects: These items retain their full value in the marketplace, and you can either trade them in for coin or use them as currency for other transactions. For exceptionally valuable treasures, the DM might require you to find a buyer in a large town or larger community first.

Trade Goods: On the borderlands, many people conduct transactions through barter. Like gems and art objects, trade goods – such as bars of iron, bags of salt, livestock, and so on – retain their full value in the market and can be used as currency.

Arms, Armour, Gear: As a general rule, weapons, armour, and other equipment fetch half their cost when sold in a market. Weapons and armour used by monsters, however, are rarely in good enough condition to sell.

Magic Items: Selling magic that you find as loot is problematic. Finding someone to buy a potion or a scroll isn’t too hard, but other items are out of the realm of most but the wealthiest nobles. Likewise, aside from a few common magic items, you won’t normally come across magic items or spells to purchase. In other words, the value of magic is far beyond simple gold and should always be treated as such.

See: D&DNext: Wealth

If you don’t run an economy in a game, it is very easy to just give Characters the value for the loot they find. Some weapons, long sword +1, considered Uncommon, are up to 500gp, which is not bad. The standard +1 that is not all that fancy or has no history may only capture 125gp, one with a history, a story or is especially attractive, could fetch up to 500gp. A +2 longsword could fetch anything from 500 to 5000gp. Many swords have added powers that may kick their value. Problem is money at hand, not many merchants can spare 5000gp, unless it is a certain resell. The average value to the PC will be half the estimated price, and even then there may be some barter, trading of supplies and the like. This will be more noticable if you are selling in smaller centres. Larger centres may have an abundance of similar items, unless the item has legend attached to it. But that is up to the PCs to discover.