It is completely up to the buyer. If he/she is interested, I am more than happy to talk about the bike as long as they are willing to learn. If they just want to buy the bike, and have an "uninterested" way of acting, then I won't force feed them information. But these things often comes down to what bike you are selling. Today I sold two bikes.
One was an old, banged ud Benotto, and the buyer had no interest in talking to me more than he had to. The other bike was a pretty pristine Italian in great condition - and it was of cause a lot more expensive that it's Italian brother. This time the buyer, who barely knew how to change gears, had some interest in gaining knowledge about the bike he was about to buy, so I explained some things to him.
So I guess it comes down to the buyers mindset, and the price. And those two things are very often connected. I have rarely sold a $600 bike to someone who have no interest in knowing how to change the gears.