Your letter does not have to say anything about your past sales.
I would state my case as to why they should talk to you in order to sell the property. Address concerns they might have. In our area, large plots of land are old farms that have been in the family for hundreds of years. These people are afraid that their precious farmland is going to be sold to a developer who's going to put 10 houses per acre and ruin the countryside. If I'm targeting that kind of seller, I'd educate them about how they can put in deed restrictions to prevent things like that from happening. I also tell them that it will be harder to sell with restrictions, but it's a way for both of us to get to our goals.
What you want to do is educate the land-owner - they may not know how much their land is worth. They may not realize that their land that backs up to that national forest with the perfect view of the mountains where no one will ever be allowed to build is ripe for a house, and will sell for a premium. I'd do a mini-CMA overview and let them know that you have interested Buyers.
If you speak to them as an individual and not as part of a mass-mailing, you'll have better results.