I actually look for horses like that. They are cheap and I like to train them into good trail horses. The owners almost always list them as "green broke" but I find that out here in the east that can mean just about anything from not halter-broke to dead broke but ornery.
Here's what I look for:
First, whether a horse is registered or not is not important for me, so I do not allow that to be used against me as a bargaining point. If a person says, "He's a purebred stallion and could be registered, but I just didn't put in the papers." as far as I'm concerned he's a "grade" gelding.
Second, I look for a horse that is easy to catch. It indicates the horse has probably not been abused. Abused horses have emotional problems just like abused humans do. It's sometimes tough to train them out of it. It might turn out to be a great horse, but it might not.
Third, I look for good conformation. Now I don't mean it has to have perfect legs or head or anything that is particular to a particular breed. It just has to look good to me. I want straight legs, no bad scars or proud flesh around the coronets and hocks, solid hooves, decent withers and back, and a nice-looking head. We all like pretty horses, but what I really want is a sound one. I like loooooong rides in the mountains and I look for horses that will stand up to long rides and hard work.
Fourth, it has to load in a trailer. I never pay the money before I have the horse in the trailer. You can't get him home if he won't load. A horse that won't load in a trailer is absolutely useless to me.
Lastly, I look for a "gentle eye". By that I mean I stay away from horses that are flighty or show white around their eyes as I handle them. Often that's why they are being sold - they are "too hot to handle". At 6-7 years of age, that is likely a pretty set trait in them. I don't worry quite as much about that in a horse that is 2-3 years old.
Just to relate a funny story about the horse in my avatar, she was being offered at $800, advertised as a green broke 6 year-old quarter horse mare, but being sold because she is a little lazy. When I arrived to look at her, the owner told me they had owned her for several years, but were not using her. She looked healthy and sound. She came right up to the owner in the corral and was easy to catch. She showed a very gentle disposition and was very personable. I liked her right off.
As I talked with the owner, the truth came out. When they rode her, she would hardly go and would walk very slowly. They didn't like that, so her husband got on the mare and tried to get her to "go" (the husband told me they like horses that "get up and go."). He booted her and she promptly bucked him off. They never rode her again. I was starting to get a better picture here. It appeared to me the horse had no training and didn't know how to do anything. They owners had no idea how to train a horse.
I used my heavy lead rope to snub her to a tree with a safety knot, then brushed her down. She was fine, even liked it. She fidgeted when I tossed my blanket onto her back, but stood when I put my saddle on her. When I began to tighten the cinch she exploded! I stepped back and watched as she jumped and bucked and plowed into the tree, finally ending up on her back with her neck stretched out and her feet in the air. Once she quit struggling, I reached over and pulled the knot out and let her up. I looked her over and made sure she was not injured and calmed her a little.
Then I walked over to the owner and told her, "I like the horse. There are several others I'm going to look at first, but I may come back for her. However, if I do, I'll be talking $300 rather than $800." The owner looked at the horse, then back to me and said, "I can do that!" The horse's antics really helped out in getting the price down where I wanted it!
After looking at several other horses, I did go back. Before I paid the $300, though, I told the owner that if she did not load in the trailer I would not buy her. The mare was reluctant, but loaded and I bought her. I've been happy with her and she's turning out to be a very good trail horse.
For her, "green broke" meant she was halter broke, but not much past that. She's had me on the ground four times as I've trained her (definitely not lazy), but she has turned out to be a very nice trail horse for my 16 year-old daughter.
Food for thought: If the ad says an experienced rider is needed, take that as gospel. It's likely an inexperienced person selling it, so they know first hand and are trying to be honest!