1. Not that I know off.
2. There is no tune with the title Benny Blanco, what you probably mean is a tune or filler which they have used in the past in their live sets. They have played this track at several occasions. If you mention which one you would like I can see if I have it and if I can upload it somewhere. At this moment I don't know where you could download any version of this link.
3. It could be useful as a trading item. Be prepared to pay between 30 and 70 Pounds Sterling though, since that is what most copies sell for. I would suggest to wait for a bargain. Be patient, otherwise you will end up paying too much for it and it's defenitely not worth the money.
4. Which Mindfields promotional copy? Remember that there are more then one version available. Versions which I know off include ten Minefields whitelabels, and fifty cassettes, but you obviously don't mean these since you wrote Mindfields. Then there is a French Mindfields promotional CDS in cardsleeve as well as a CDR on Maverick. Personally I only own the French promotional copy. I have seen it for sale at various locations, but prices are too high in general; from 20 to 40 Pounds Sterling.
5. There is one Serial Thriller promotional copy. If you would have browsed to Kotta's Prodigy web site you would have seen it for sure, but I have no doubt that it is mention on this web site as well. It is an Australian Sony release and features one track, Serial Thrilla. The title of the item is spelled wrong by the way and reads Thriller instead of Thrilla. The item contains the album version, just like the French Mindfields promo contains the album version of Mindfields.
6. You cannot trust them, it is all a big risc. Nevertheless I know a lot of people including myself who are willing to take that risc. Personally I have lost at least $375, but probably more at eBay, Yahoo! and companies like those due to people frauding you. You can use the feedback function at most auction web sites to see how trust worthy a seller or buyer is, but this is in no way a garantue, please remember that.