If you guys really want a sensitive beam scale, then you may be going about it all wrong. This is from my own personal experience. My experience was that I made sure to buy & have 5 scales on hand to compare them. Everything is relative, and without being able to compare scales against one another, it is difficult to know if you have the most sensitive and the most consistent. I started with an old 1970's 10-10 scale (which is a version of the M5) that I owned since the 1970's. I then bought an additional 4 used M5 scales. (ie; they are no longer made and you can only buy them used). I never knew how poor that 10-10 scale was for precision handloading that I had been using for so many years, until I compared it to other scales.
Scott Parker will most likely improve your USA, 10-10 or M5 scale, (don't bother with China or Mexico made) but there is no guarantee that he will surely turn your scale into a precision marvel. This was the case with my old 10-10. For my precision shooting needs it went from mediocre consistency to "Ok" after Parker tuned it. (noticeable improvement, but not great)
So then over a couple of weeks of shopping, I proceeded to buy M-5 scales at Ebay and do a close study & comparison of them against each other. If it takes your buying 1, 2, 3, or 4 of them, then so be it. (just as I had done) Have them on hand at the same time, and compare them to find the scale that has the best single kernel sensitivity and consistency too. You then later sell what you don't need.
In your work with used scales, did you do any knife edge sharpening. I have done that on every scale that I have worked with, with significant improvement in sensitivity. Also, it has been my experience that using a web cam to read a scale is a big improvement, both because of the magnification and the elimination of parallax issues.
As stated, I first started off with my USA made 10-10 scale and sent it to Scott Parker. I got it back from him, and noticed an improvement but I wasn't so sure to what degree.(...because I had nothing to compare it to). So I proceeded to look for and buy 3 used M5 scales on Ebay and another 1 M5 on Gunbroker. This is how I determined that my Scott Parker 10-10 scale was improved but still did not compare to a fine working M5 scale. After much testing and comparison of the 5 scales, I kept the two most sensitive M5 scales and sold the other 3 scales including my 10-10. The 2 M5 scales I kept are capable of sensitivity to a single IMR3031 kernel, and even still somewhat sensitive to the even much smaller single kernel of RL7 powder. (it's more like a granule instead of a kernel). So now you're going to ask me, "Then you knew you that you were selling 3 other dud scales you didn't want to other people ?!!" And my answer to that is an emphatic "NO!! When I sold each of the other 3 scales, I divulged the full potential by showing photos of two IMR 3031 kernels, or three RL7 kernels that each of those 3 scales were still very capable of weighing. Those 3 scales were happily bought by others, and I never heard a complaint from those buyers or saw my scales being resold in classifieds again. The fact of the matter is that many handloaders do not seek or require that their scale is capable of single kernel sensitivity for their 45/70 or M4 rifles, or SigP365 pistol for range shooting. It is a relief for many buyers to see a demo photo of 2 kernels of IMR3031 powder by a seller and know that the scale they are buying from him is capable of that accuracy on a consistent basis and will surpass the buyer's demands for the style of reloading and shooting he intends do. So, when it's time for you to resell whatever scale(s) that don't suit you, just be open and honest about the capability of what you're offering, and somebody will happily want it and buy it from you quickly.