When she had explained why she was there and she was authorized to offer them a "job", they had given her their undivided attention. Instead of just hunting the supernatural that were causing problems, they would help  her reclaim magical artifacts that had fallen into muggle hands. Dean and Sam hadn’t really thought it was a problem until she showed them some of the files the Ministry had. Once they were shown just how dangerous and, sadly, often these artifacts reigned havoc, they were onboard with helping. Oh, and finally being paid for what they did, magical or supernatural, was icing on the cake.