Heres the link http://cgi.ebay.com/ebaymotors...ZWDVW. This dealer seems reputable, is this legit? I know nothing about interchanging LSDs; can this be installed in my 99 Si tranny? What is needed?

 

The auction looks legit, the price is legit thats for sure.
But to answer your question, yes they are interchangeable.
People do it quite often in place of a Quaife LSD.

Also, everything you need to know about the LSD install:
https://honda-tech.com/zerothread?id=795700

Good luck.

Edit: Sorry that link has dead pictures use this one it's the same thing:
http://www.xm-parts.com/articles/hgear/gear.htm

 

wow LSD for under 300...beats the 1000 for the quaife...

is there any performance difference between the two?

if there isn't i just might buy one of these for my 99 si tranny.

 

I was told this is incorrect information after I purchased this LSD! Luckily TSR, my performance shop, has enough experience to retrofit the LSD to my tranny.