Well, firstly you have the charger - which is 220v and will not worth without a voltage converter in the US (which uses 110v)..Secondly, the UK PSTN base won't work and the buyer will have to use a US spec one.
However, if he changes the PSTN to a US version and gets a voltage converter or a new charger - he can (as far as i am aware) register your BeoCom 2 handset to his new set-up.
Lee
BeoWorld - Everything Bang & Olufsen