I'd be concerned about getting a good sample. You take a bit off the top and who is to say that it's at all representative of the other 499 pounds.
I might take a sample by sticking a pipe deep into the material to hopefully get a nice selection of any possible strata. Then take this sample and either have it assayed, or do some rough testing on it.
Leach the sample in 10:1 HCl:30% H2O2 at room temperature and this will pretty quickly dissolve the palladium and platinum. Ground up material might contain a lot of base metal contamination, so be careful, as the reaction might get out of hand - I'd let the material sit for a bit in straight HCl to dissolve any base metals.
Usually I can get a reasonable idea of the grade of the material by the colour that the peroxide leach solution develops, assuming base metals don't change the colour too much.
A nice deep red or brown colour after the solution clarifies is a very good sign. Orange is decent. Yellow is usually pretty weak. I'd confirm this with stannous chloride testing, looking for very deep stannous reactions. At this point depending on the results I might feel comfortable paying a low price for the material on the spot, without a real assay. Even if I thought it was excellent material, I don't think it would be advisable to pay more than $8-10 a pound. With a rich looking solution, and deep stannous colours, you'd be unlikely, in my opinion, to come up on the losing end at that sort of price, and the seller should be satisfied.
You could get a real assay, but in between sampling and your receiving the assay who has possession of the 500lbs and will it be tampered with? Maybe you could put seals on the container, if the seller is agreeable to you holding onto the material.
Whoever buys the material from you will want to have their own assay done anyways. At which point your own earlier assay is only a way to tell if you got burned.
As far as buyers for this sort of quanitity, I remember EDI gold said they could do this. They quoted reasonable payouts too.