A4000 keyboards are not worth £30
If you fitted an IDE-CF adapter, a 2Gig (or more card) and pre-installed Workbench and some software I am sure you'd get well over £250
Especially if you included an RGB SCART cable.
I paid £65 recently for an untested, sold as seen, non-pro SX32. (Which may or may not give you help with your pricing).
The main problem with SX32-MK1 (and SX32-Pro) is that they can be a loose fit on the CD32 edge connector and cause problems. If you can testify that you've never had any problems (perhaps sell your CD32 with the SX32-Pro to ensure a known fit) then the price will reflect this.