What are the differences between the HK USP9 and USP SD9 besides the Threaded BBL and adj sights?

That is about it. The SD does not have a match trigger like the USP Tactical. And I do not know if the SD has a O ring or not.

how much would a used USP 9mm bbl go for?

I'm almost positive that the SD doesn't have an o -ring

you are correct mine does'nt have an o ring

You will find some sellers on GunsAmerica saying they have o-rings but they are flat out lying.  The 9SD does not have an o-ring nor does it have any match trigger parts even though on HK-USA's web page they use to say it does.  That was mistake and if it is still there saying that it is a mistake not yet corrected.
A used USP 9mm barrel IMO is worth no more than $75.  Since a used barrel has a serial number on it IMO it is worth less than a HK replacement barrel that will have no serial number.  Again, IMO a barrel with a mis-matched serial number looks a lot worse than one with no serial number at all.
Lastly, IMO it is better to hang on to a used USP barrel that has a serial number that will match your pistol than to sell it off.  Later on a buyer of the pistol will pay quite a bit more for the original barrel than if it only comes with an un-serial numbered barrel (replacement 9SD barrel) or an aftermarket barrel.  Most HK buyers want their parts to match.

i thought there was some kind of difference in the slide that makes it more suitable for a suppressor.

Quoted: i thought there was some kind of difference in the slide that makes it more suitable for a suppressor.

Nope.