has to be witnessed by my inspector
The weighing may be performed by the owner/builder with calibrated scales in a way that is acceptable to the inspector and the LAA. This may be by the trust the inspector has in the owner/builder and/or the quality of the person or company employed by the builder owner to perform this task and the production of the relevant paperwork on the approved forms.
My own scales are calibrated by a company with a traceable national standard and also I have a calibrated set of weights (up to 350kg) that I am able to use in-between 2 yearly calibrations to confirm that the scales are still within tolerance. Once at an airfield prior to weighing a gross error check is also performed on each scale to ensure nothing has gone awry during travel to site.
For an LAA aircraft the results are written up onto the approved LAA forms and various loading examples are calculated using loadings called up in the BCAR rules.
There is nothing inherently wrong with using a set of bathroom scales provided there is a means of calibrating them and providing you level the aircraft correctly for each reading, thats not just lifting up each wheel and plopping the wheel on the scales in turn !
Enjoy everything - Life is Short.