I am selling a 2774 sq ft home. Built in 1964. The new buyer wants to upgrade breaker box from 100amp to 200amp. My electrician said 150amp is fine. Your opinion.

Are you saying the buyer want you to do the upgrade prior to the sale? That would be ridiculous in my book. Sorry but not knowing what you have now or what the buyer hopes to add there is no telling what amps you need.

Your electrician needs to do a load calculation to determine breaker box size. The buyer may be adding a hot tub, a home theater, work shop with large power tools, etc. Without a load calc, just saying 150 amps is fine is not correct.

Ball... 1 I agree, upgrading before the sale is strange, but I've seen it done. Of course, the upgrade would be included in the purchase price, and only if purchase contract has been signed stating such. On the other hand, if the upgrade is a condition for the sale, the seller would give the buyer a credit for the cost of the upgrade, but the seller runs the risk of giving the credit and the work never gets done.

IMO asking to have a service upgraded to make a sale go through is akin to extortion. This is a very typical tactic that buyers use via their home inspector minions. The HI says that a 100A service is not "up to today's standards" so the buyers use that as leverage to get a free upgrade out of it.

Also, a load calc is completely unnecessary. If you get strong armed into this just to the 200A. There is only a negligible difference in material cost and no difference in labor. 150A services are a completely pointless thing in today's world.
Also, also, WHO CARES what the new owner might do. They are getting a free service upgrade, let them worry about if they don't have enough power later on.