It is definitely the buyer's job to perform due diligence and calculate their own repair costs before buying. Any flipper who relies on a wholesaler's estimate without calculating their own deserves what they get. What the wholesaler gives you is only his or her own approximation of repair costs. And lets face it, true repair costs can be difficult to estimate.
I think where wholesalers get a bad reputation is when they produce thin deals with very clearly inaccurate estimates. The flippers aren't complaining so much about getting stuck with a bad deal (because the good ones have done their own due diligence and avoided the bad deals) but that so much of their time is wasted with amateur wholesalers trying to talk to them about deals that just really aren't deals.
By the way, these poor deals are also a hazard for wholesalers. I'm a wholesaler in Northern Virginia and I get a lot of calls from other "wholesalers" trying to pawn off thin deals. "Hustle" is great, but it needs to be tied to knowledge and judgment.
So, in the end, wholesalers are like anyone else (including house flippers) - there are good ones and bad ones and the reputation of the bad ones tars both good and bad. But you will find that many or even most successful house flippers have located one or two good wholesalers and buy the majority of their inventory from these folks. They've found who they can trust and the two develop a close business relationship. As a result, they less frequently deal with others. So the good wholesalers often pair off with a small number of buyers and the poor ones remain free in the market along with the new ones. The result is a reputation that's worse than it should be.
Bottom line: Yes there are many successful (and respectable!) wholesalers.
