E&O is errors and omissions coverage. It responds to claims that your professional work was wrong and cost the client money, which is different from general liability, and general liability responds to bodily injury and property damage. A buyer asking for it is treating you as a professional service provider, which is what you're proposing to be.
What a claim looks like here is usually one of two things. You gave a rent estimate or an after repair value the client relied on, they bought, and the real number was far off. Or you told them something about the property, square footage, a permit, the condition of a system, that turned out to be wrong. An out of state buyer who never walks the house is relying on your description almost entirely, which is exactly why he's asking.
On cost, small service operations in this space are commonly quoted in the low thousands a year, and some carriers won't write wholesaling or unlicensed sourcing at all, so plan on shopping it through a broker who knows the space rather than filling in one online form. An insurance broker is the right person for what's actually available to you.
The indemnity language matters at least as much as the policy. Broad indemnity can require you to cover his losses and his legal costs for things well beyond your own mistakes, and your policy may not respond to a liability you agreed to by contract. Have a lawyer read it. Separately, put a written limitation on your own estimates into your agreement, saying your values are opinions and the buyer is responsible for their own inspection and underwriting. That sentence prevents more claims than the policy does.
