OK, I will send the bass to mainland Europe.
Delivery cost: UPS charge just over £50 economy while DPD charge about half that (but I don't think they are as reliable as UPS).
Total cost: I would want £999 net of all bank charges for the bass, plus delivery at cost (agreed with the buyer as above). Approx. £1,050 net assuming UPS is the courier.
Also, I realise Brexit means the buyer would be charged import/duty on the declared value but I am afraid I will not bend the rules to save the buyer money. What I get paid is what I will declare on the customs forms.