Your local library might have a copy of "Obrien's Collecting Toy Trains" 6th edition. It has a section on Marx trains and since it was printed in 2006 it is fairly current.

If you want to sell your trains, I can make two recommendations, both are dealers with whom I've done business and I trust them both:

Charles Siegle of choochooauctions.com or traincity.com

Ray Ellen of Vienna Station at trainmarket.com

I do not collect Marx myself, but I have not noticed a lot of Marx on Choo Choo Auctions or Train City. There is a lot of Marx listed on the Vienna Station web site. Either dealer would be fair and honest with you.