If I bought all my DAO tokens with the same account, will I be able to sell only a portion of them on a marketplace?
After the creation period ends on May 28th, you will be able to send DAO Tokens, similar to how you send ETH. You can send your entire amount of DAO Tokens or just a portion of them.
It should be noted that with The DAO Tokens specifically, you cannot send the tokens if they are currently "tied up" in a vote. This prevents a single token from voting more than once.
Once you are able to send, you will be able to exchange / sell / trade on exchanges that support this Token. It will (most likely) be similar to how you exchange ETH, BTC, or any other alt coin. You will need to send the DAO Tokens to the exchange and then open a sell or buy order at a price, etc.
Gatecoin was going to handle DAO Tokens, however, due to their recent hack, it is unknown if they will be up and running by May 28th. We will have to wait a couple more weeks to see what exchanges support The DAO Token.
BTC Markets will support trading for ‘The DAO’ tokens on the 28/05/2016: the same day that the creation phase of ‘The DAO’ ends. The platform will initially go live with DAO/ETH and DAO/BTC markets. After the creation phase it’s possible for token holders to transfer ownership and deposit tokens onto BTC Markets for trade.
Tutorials for MIST and MyEtherWallet are being developed so users have a clear understanding on how to transfer tokens
About DAO https://btcmarkets.net/about-dao
Bittrex seems to support them, under ticker DAO. You can't trade yet (until 5/28). But you can "buy in" with ether.