What are your goals with this horse? I think the real question is what the market is for a horse that meets your goals. And where this mare fits in that market – high end; low end; middle.
If you are buying intending to stay comfortably at 2nd level, that is one thing. If you want to get her back to 3rd and show competitively at that level, that could be another.
If you can say honestly to the seller “I would not be buying her as a 3rd level horse, because at her age and the time since her last competition at that level, I can’t count on that”, it might soften the sellers’ expectations.
Especially if there are no other prospective buyers wanting her for more. If other people are interested in her as a legit 3rd level horse, that is a different price picture.
Realistic expectations have a great deal to do with what a horse is worth to you, as well as to the seller. What you want to do with the horse tells you what price market you may be in. What else you can realistically buy for the same price. How those other options compare to this one.
Any horse is worth what someone will pay for it. If no one else is as interested as you are, that gives you more leverage over what you think she is worth, for your own riding goals. That is, how much you are willing to offer, in comparison with your other choices. And what you are not willing to pay to have this particular horse, because you could get more for that higher price.
Remembering that at her age, every subsequent sale may be for less than the last one. Unless she starts setting the world on fire in the show ring.
I hope that makes sense.