Can you link the ad? The desire HD is a fairly old phone so the price would seem reasonable, though I would still steer clear. Unless it's sealed in original packaging and has never been used, you have no idea would could be wrong with it or what their reason for selling the device is. Craigslist and Ebay in particular are hotbeds for scammers.
Also, those offering "unlocking services" are blatantly breaking the law. If you buy a device outright from any carrier, they are legally obligated to unlock the device. Same applies to any subsidized phone for which you have fulfilled all terms of the contract in their entirety. These rules are all clarified in my sticky about unlocking at the top of this sub forum. Those who offer assistance with unlocking your device, are specifically targeting people who have already committed breach of contract and want to try and take their subsidized phone, to another carrier.
This thread is a good example of what
not to do.