To me the better question is what are the advantages of buying a started dog. You can do a search and find previous discussions on this topic. What type of dog do you want to run. When purchasing a started dog at say 8-10 months of age or beyond you can start to tell if the dog is a team player, fire breather, type of line manners, whether he/she is a thinker, will go/look long, etc. I would think back to some of your previous HT that you've run and consider the traits in the dogs you felt helped make the dog successful. For me I like a dog with a lot of go, one that shows early maturation and focus and is a thinker.
The biggest thing is how well do you know the person you're dealing with. Many people think a started/finished dog means one thing when you think it's another... Make sure they shoot a picture of the hips and elbows also before you buy.
I have 3 available. They are not on my web-site. 2 are new to running blinds and fresh out of the yard and the third just needs some more polish on his blinds,, All are extremely good markers and will make wonderful hunt test dogs.
Think of the things you don't want to deal with, like noise, kennel pig, temperament issues, gets along with other dogs, raised in a kennel or house, make a list of what you just will not compromise on in that regard. Especially ask about health, not just clearances but everything up to date including current clear heartworm test, allergies, CCL ruptures, any other surgeries, etc. Started dogs can be good, or they can bring you some real problems and not just in training or ability. I got one that was a rock eater. Keep in mind, some aren't going to volunteer anything negative and you better ask as many specifics as you can. Apparently rock eating didn't fall under my question about any quirks or other issues the dog might have.
Why is the person selling the dog.
What is the dog's training level and how experienced is the trainer, what's it been exposed to, strong in the water, had birds/cripples, gunfire, decoys, the whole HT atmosphere in group training, or does it freak out at a birdboy wearing a hat and never saw decoys or a holding blind before? If you can have someone assess the dog in person, great, if not, better hope the seller has a good rep or can at least provide you with some references.