^lol.
Most dealers I know have tested the product themselves, so they really have no excuse. When I used to drop pills/ecstasy my pill dealer would actually go out of her way the night I bought the substance off her to ask me what I thought of the pills. But, she was a cruisy chick that didn't seem greedy like alot of dealers can be.
As for things like meth or Heroin. Well, I suppose you really have to just accept the fact that there's extremely unpure stuff going around. Alot of dealers I know don't even bother cutting their gear anyways... they just buy it as is and take the risk that it could be shitty quality. Just like the guy above them and the guy above them and so on...
There are some genuine people in the illicit drug trade. But, unfortunately alot of them consider it deal done once the transactions been made.
I suggest that if you're not happy with your product to just ring your dealer up or email him/her saying, politely of course... that you weren't too satisfied with the quality of the product.
But, I wouldn't be asking for refunds, unless you want to get laughed at or get told to fuck off!