Well, I have bought alot of stuff at flea markets, and you know how dirty some of those things can get.

One thing that I use all the time and it has worked great...

Magic Eraser.

It's a Mr. Clean product, and its just a white eraser looking sponge and it cleans all plastics perfectly.

It also got some marker off a label (but be careful it could remove some of the color off of label but it worked for me) Partly removes marker off of plastic, and removes most marks and dirt. Try it out.