I have someone from England interested in one of my complete systems. Chip is clipped.

Will it take only an adapter so that the USA power supply will fit his electrical socket? Or is there something more fundamental that would be a problem?

Got an answer elsewhere for those curious:

Xevious: If I understand your question correctly your customer needs an UK A/C adapter (220V -> 9V) to use the NES. UK is 220V and US is 110V so if he/she just use only an UK socket adapter on an US A/C adapter the NES will kickstart and die.