Well, I'm not a good one to ask because I don't like to sell much of anything. I have to say that no, I won't sell my best broodmares--and by "best" I mean "best for me". I have sold a couple that didn't work for me--not because of the quality they produced, but because they needed to be bred to a smaller stallion than what we were using here.
Here's what I found when we were buying broodmares though: I was also of the opinion that most people aren't likely to be selling their best broodmares. However....what is best for them may not be what would be best for me--and some of the mares that don't work for them may work very well for me. For instance, one breeder was selling off all her over division mares. So, I bought two of them, because I liked the mares, I liked their breeding, and I wanted over division mares. Indeed, they both produced well for us.
In another case, a breeder had bought this particular mare for resale. I really liked the mare--she's nothing fantastic, but there was something about her...turns out that she produces lovely foals (I consider her to be our best broodmare)--she's been bred to several different stallions & each time she has produced a quality foal. To this day I don't think the person I bought her from likes that mare, but she's a mare that we won't part with. She's got beautiful movement, produces quality foals and is an outstanding mom.
I have bought other mares that the seller had a personality conflict with. That was no reflection on the mares' quality of produce record, just simply that their personalities didn't blend well with those of their owners--and so they were willing to sell the mares reasonable just so they didn't have to deal with them any more. I can get along with just about any of them, and the mares haven't been a problem to me.
And of course sometimes a particular breeder just doesn't have the right stallion to make the right cross on a particular mare--in that situation the mare isn't a good producer for that breeder, and so they put it up for sale. Someone else--you, me--may come along & buy that mare & take her home & breed her to another stallion, and get a wonderful foal from that mare.
So, while I do believe that breeders tend to not sell their best broodmares, I also believe that some of the mares those breeders are selling are well worth buying!