He must have either been optimistic or hallucinating. I've never had anything 'insulting' offered value wise, just not some things I would never want. What bass did you value at £200 ? Unless of course revealing this would reveal the identity of the offerer .
But i've also had the opposite happen, I had a 1980 Ibanez Musician MC900 (bought for £250) which was in reasonable condition. I was offered a straight swap for a 1996 Stingray (worth £750) about 4 weeks later, without me even saying it was for sale to start with ! Needless to say I snapped his hand off, have since traded several times for equal value.