On one of my wholesale deals, i was negotiating the price with the seller, who needed to sell for enough money to pay for his mom's nursing home. Based on what he said the monthly cost was, I realized that his asking price was about $2k too low. I pointed this out to him and raised the purchase price $2000 higher than what he had said he would accept.
I have helped sellers move, I once tracked down an out of business mortgage company to get a paid in full letter for a seller, I helped relocate a tenant, all to get the deal closed, but also doing work that the seller wouldn't/couldn't do themselves or else they would have been able to sell their house the normal way. 2 of my sellers were about to be foreclosed on and get nothing but were able to get some cash out instead.
No, I don't explain that I won't be the end buyer and yes, I had to cancel a contract once. But when I sign with the seller, I explain that I have a 10 day due diligence period during which I can cancel if something unexpected comes up (not being able to find an end buyer would be unexpected since if I didn't expect to be able to find an end buyer, I wouldn't make the deal). Most realize that this carries no more risk than a conventional buyer with a financing and inspection contingency.
I don't know about other wholesalers but I work hard to find motivated sellers and then I work twice as hard to make sure my deals close. I don't have to justify my fee to anyone, I earn it. Sorry, not sorry.