I used to do a lot of the same type of work. Man can you ever run across some NASTY repo houses. The few clean outs I got, paid good, but the inside was way nasty.
I worked for a broker so I always got paid in 30, then they were the ones to worry with collecting from the next guys up the ladder.
My advice to you is to draw up a contract, and if they do not sign then no work. You can advise them before hand that there is a new policy and it will be starting on so and so date. I have said it before and will say it again. It is BS for the big commercial companies to hold your money for more than 30. The ONLY reason they do this is to make more money off your money as intrest at their bank. Some people will argue that it takes that long for paper work to be processed. Too bad, pay up. Even the government in not that bad at paying. National Weather Service pays in about 15 days, direct deposit to my bank.