With HUD properties doing the double close is difficult if not impossible. We have purchased several HUD properties. Most of the HUD properties we see are not sold at enough of a discount to meet our criteria, but occassionally one be available at enough of a discount. We purchased two last week, but those were the first two in many months. The challenge with HUD is as Tom said - you put down earnest money up front, demonstrate proof of funds and must take title in the name that is on the original contract. The contract is not assignable. In addition HUD specifies which title company to use and they are not at all flexible.
In our case we pay cash for the properties, then rehab as necessary and either list back on the MLS for re-sale or keep as a rental. You have to have the cash (or a loan) at closing.
I have heard or people discuss the idea of setting up an LLC just for the purpose of purchasing a HUD, then after getting the HUD under contract transfer the ownership of the LLC on the day of closing to the person who put up the money to buy the house. Basically trying to create your own double close. It creates a lot of risk for the person putting up the money however and I would not personally recommend it.
As for the second part of the question, yes we have had success at selling proerties we purchased from HUD to both other investors and to end buyers. It basically comes down to buying the right house at the right price as with any other transaction. HUD tends to follow a declining discount schedule. They give preference to owner occupants, but for those that are not purchased by owner occupants the properties can be picked up at a discount. Initially they can be generally be purchased at 90% of HUD's list price. After a defined period of time HUD will drop the price 10% and re-list the property for 5 days with another owner occupant preference. Once this period is over it can now be purchased at a larger discount and so on.
You don't need an agent to search for them but you do have to have an agent who is signed with HUD submit the bids for you.