@Steff I'll start by saying - What a bass! If I had the £££ I would have been PM'ing you long before now.
I think there's 3 issues at play here for you:
1. Some people see non-reversible modifications as a turn off, regardless of the parts & costs involved,
2. No case/gig bag,
3. Collection/Local Delivery only
IMO if you can address points 2 & 3 you'll reach a much wider buyer base and I reckon it would shift.
GLWTS
Cheers, Jon