I looked around and found a French forum where someone asked the exact same question as you, and got pretty much the same answer : worth about 50€. He (user name KoZak) sold it for 60€ in the end.
Here's the link, it's all on the first page :
http://www.automodelisme.com/index.p...&topic=38602.0
This said, some RTRs are pretty cheap, so you might still find someone who's into vintage cars who would be willing to swap.
Otherwise, why don't you put it together and use it? It's a great buggy.
You wrote that you found spares on ebay? I only found some for MBX4/5/6, so would you mind posting/PM the links?
Thanks!
J.