Now the buyer wants to know if it's possible to assign the IP address of their current site to the domain that's being sold so that when the spiders visit, they will index their (buyer's pages). I have no idea if this is possible.
Hope this makes sense. Thanks in advance.
You could do it the other way around - transfer the domain ownership to the buyers and have their hosts point it at their current site, but you'd be running into duplicate content issues (2 domains pointing at the same site).
You plan of redirects sounds a better bet, although you might consider using the original domain as a seperate doorway site feeding traffic to the new one.