When you burn hydrocarbons-H(gasoline) using Oxygen-O, one of the by products of this burning is H2O(water).
So exhaust systems always have water flowing thru them, this is why they rust from the inside out.
Stainless Steel doesn't rust, which is why it is often used as a "lifetime" exhaust tubing.
But there are different grades of stainless steel, even a lower grade shouldn't rust very fast but can discolor and look rusty.
And in salty conditions it will discolor even faster, i.e. on boats and vehicles using salted roads.
A used stainless steel exhaust system should be fine to use, if it fits then go for it.
If it still looks good then you will know it is a good grade of stainless steel.
As said there are different grades of stainless steel, same as all metals, the composition sets the best use.
A 0 rust stainless steel may not handle repeated heating and cooling well, too brittle, so can't be used for exhaust.
I do not know about metallurgy for stainless steel, but I am sure you can look it up.
Mufflers and Cat converters will still rust out, so just stainless steel tubing is all you are buying, or should be buying.
If they are including the muffler fine, but as far as I know it isn't "legal" to sell used Cats, so no charge for that, just buying exhaust as is
Last edited by RonD; 08-09-2016 at 02:42 PM.