The price is fair for mare. Does it include the foal? In some disciplines the sale of the mare does not include the foal. I know this sounds weird (it certainly did to me when I firse experienced it) so ask the question.
My major concern is the training and age. The age is not stated. Do you know how to age a horse from teeth? If not Google it and you'll find a number of good explanations.
Then there's the training issue. The ad says mare was "used to ride and drive prior to our aquiring her." This can be read in multiple ways. I'd ask a lot of questions and then require that the seller ride the horse and demonstrate all three gaits.
In truth you're not going to get a lot of trining in a $1000 horse, but this is also the kind of horse that can put your health insurance to the test. Be careful, ask questions, and require demonstrations.
Good luck if you decide to go forward.