Rust prone carts can be dealt with a number of ways. Basically, keep it off of concrete, or, if on concrete, put a grill matt down underneath it. Short version is that concrete will give off fumes that will corrode the bottom of the cart.
$350 is on the upper end for that 3 burner as it doesn't have a side burner (but does have the sear burner). I'd still see if they would take $300 for it. If you have the coin and can use the space, that Summit pricing isn't very far off as it was close to $2K when new. Just be sure that it's in great condition.