Welcome, Raymond!

I've moved your post to the more appropriate optimization forum.
1. Is it very beneficial in terms of Rankings to have one's products listed?
If you're talking about Froogle, then it's beneficial because of the traffic it brings in it's own right. If you're simply talking about having product pages listed in Google, then it's beneficial because it gives your site more chances of being found.
2. Are big products like trucks or portable buildings suitable?
Again, if you're talking about Froogle, you should check with the Froogle guidelines. I'm sure they'll be happy to let you know what's acceptable.
Good luck!

Jill