Many sellers are being pro-active and offering the home warranty up front as part of the enticement to purchase their home. This is a wise action for sellers, in particular those with older homes or homes with older mechanicals. These warranties will not cover pre-existing conditions or normal wear and tear - but can certainly save some big bucks when experiencing a mechanical failure with your HVAC, water heater or covered appliances.
Sellers can also get coverage during their listing period (usually for 180 days). Be sure to check the policies for coverage - but most of my corporate sellers now offer home warranties to buyers and also have coverage during the listing period. The listing coverage is at no cost - but obligates the seller to purchase a home warranty for the buyer. This coverage has saved my corporate sellers a lot of money and should be seriously considered if you are thinking about offering a home warranty as an incentive.
There are several providers - so be sure to read all the fine print as to deductibles, what is covered and what is excluded. American Home Shield (AHS) is highly recognized and is used by my corporate clients. I have personal experience with these listings and have replaced many garbage disposals, water heaters, blower motors, etc for sellers with only a $60 deductible.