Will a 01 boxster S exhaust muffler bolt right onto my 02 base boxster.
 

Seen a used one for sale on Craigslist $150obo. Any thoughts?

No straight bolt on, different mounting points

I'm pretty sure an 01s muffler will bolt right onto an 02 base. The mounting points are exactly the same between the two cars and the muffler is the same as well (only difference being the twin tips).

It's my understanding that any oem muffler between '00-'04 is interchangeable . In fact, I believe that with a simple bracket change, a '99 2.7 can accommodate a '00-'04 S muffler .

Are they different mufflers and how much different is the sound?

I have 01, non-s. Purchased the car with B&B muffler. ran great and had great sound. little drone at 2600 to 3000. muffler baffle broke, replaced it with 02 S muffler. all mounting points OK.
the S muffler is quiet and no drone.

Yes, just like everyone has said... Straight bolt on, mounting points are the same.

As far as I know, for those years, 00-02, they Base and S are interchangeable. The only difference being single oval tip vs twin tip. And for 03-04, the difference between the base and S is that the (single and twin) tips are removable.