Don't seller inspections take too much energy to sell to make them profitable for the inspector?

Perhaps.  But not when the inspector takes into account the marketing benefit of having a samples of his/her product (the report) being passed out to agents and potential buyers who are looking to buy now in the inspector's own local market, not to mention the seller who is likely moving locally and in need of an inspector, plus the additional chance of re-inspection work being generated for the inspector.