I like your in-Lightroom alternative John! Might give that a try. I suppose one could stack the Sale images together to keep it neat.
I do this in Aperture. When an Image is ready for sale, I color-code it blue (these labels show in the Image Browser, and are even retained in the OS file system when a new file is created by exporting from the database) and add a custom metadata field containing 10 lines. I sell my fine art prints in editions limited to ten, and simply put the buyer's name on the line with the number in the edition of the purchased print.
I put all buyers in my Contacts program (an electronic Rolodex that comes with Apple computers), and flag them to show in my "Buyers" sub-list.
I have a Smart Album in Aperture that shows Images with Blue labels that are not sold-out. This is my current "For Sale" catalog.
It's not a good system if you are selling a lot of prints, but for me it works well.
When I was painting, I set up an Access database and tracked buyers, sales, costs, and lots more. It was overkill.
As with any data acquisition tool, the better you define your needs at the outset, the sharper your tool will be, and the easier to use.
I don't use Lightroom, but I imagine it has custom metadata fields. I have set up Aperture Libraries that allow artists to track what's been printed, where it is stored, when it was sold, for how much, and to whom. The drawback is that one can't use it for arithmetic. "Total sales?" — no idea. "How many unsold prints should be in flat-file drawer #3?" — no problem.