These are the most popular sites that accept and trade BTCs. For a full list check the wiki.
• Mt. Gox
Mt. Gox is pretty much the place to look for Bitcoin rates, and almost every site bases the Bitcoin pricing around it. Mt Gox allows you to trade US Dollars (USD) for Bitcoins (BTC) or Bitcoins for US Dollars.
The website interface works pretty much like the Bitcoin Market (explained below) except it’s not a peer to peer trade: while you are still trading with other users of Mt Gox all currencies are deposited to Mt. Gox and the site will manage the trades automatically, which means you can buy or sell with confidence.
To trade at Mt. Gox, you have to make an Account, add some funds, buy or sell Bitcoins and then withdraw your converted funds, as simple as that.
• Bitcoin Market
In the Bitcoin Market (BCM), you basically put up a certain amount of BTC for sale, and then another user will buy them, and then send you Currency for them. PayPal USD is really the most active traded currency on BCM right now, but there are others, like LibertyReserve or MoneyBookers. The trades are peer to peer (so there are no fees, but there is a bit more of risk), rates are almost always higher than Mt. Gox and is usually very quick.
Once logged in you have to set up your payment processors. Go to the “Accounts” tab.
Under BTC click “Payment Processors” then click “BitcoinClient” and enter your Bitcoin address.
Go back to the “Accounts” tab, and under “USD” click “Payment Processors” then click “PayPal” and put your PayPal account address there.
Go back to the “Accounts” tab, and under “BTC” click “Deposit”. Select “BitcoinClient” from the list.
Enter your Bitcoin client and send the amount of BTCs you want to sell to the address they point out.
In less than 10 – 15 minutes, your BTC balance on your BitcoinMarket account will update. (In the following example 0.99 BTC was sent to the BitcoinMarket address).
Now you are ready to sell some BTC. Go to the “My Orders”, click “Open A New Order”, and fill out the form.
In this case, you would create an order to sell 0.99 BTC for 30 or more PPUSD (PayPal US Dollars). 30 as “Limit Price” means you wouldn’t accept less than 30.0 PPUSD for your 0.99 BTC. Whenever someone created an order to buy BTC for more than 30 PPUSD or someone clicked directly on your posted order to buy, you would have a deal.
If your BTCs are bought, under “My Orders”, “Sales”, the respective order will appear as “Filled”.
Afterwards, the buyer should send the requested money to your PayPal account and then receive your confirmation in order to get the BTCs from the BitcoinMarket transferred to his Bitcoin client. If he never sends the money, don’t worry, your sale will expire, the deal will be over, and you can open a new order to sell those BTCs, because your BTCs on the BitcoinMarket account remain intact.
Only after receiving the money to your PayPal account successfully, click “Confirm”. After that, you are enabled to rate your transaction in the “Ratings” tab.
• SpendBitcoins
SpendBitcoins is “the place to spend Bitcoins”. You can buy anything at Newegg, Amazon and all major online shops with Bitcoins. They also offer exchanging bitcoins with PayPal US dollars. Rate is based on 24 hour Mt. Gox average.
It’s really simple: just visit the website you want to shop on, mark the item you want, proceed to checkout and then complete every step up to and including entering a payment method. The purpose is to get the order total, including tax and shipping. When you have that, email spendbicoins.com with the exact amount of money you need. In less than 10 hours, they will reply back with a Bitcoin address for you to send BTC and the exact amount, based on their rate (24 hour Mt. Gox average), you have to deposit. After you deposit the BTC amount, you ‘ll have to wait at most another 10 hours and you will receive an Amazon gift card of the exact value requested to the email you used to make the original request. You can then go shopping with that gift card that covers the order you want. This site has very good feedback.