The neighbor is tearing their house down and re-building. They are asking for an easement to put the elec. Lines in the ground (10-15 feet into our property). Our house is on the market to be sold. Should we say no?

Late answer - but power lines run underground are generally run along property lines. The Power company shouldn't have to come 10 to 15 feet into your property.

All that said being a good neighbor is generally the right thing to do but exhaust the other options first