I found a Detroit front locker for a Tracker, locally. Its used, the guy said it fits the front of a Tracker or Samurai. Does a Tracker front locker also fit a Samurai? Also what should I look for when buying a used locker?

Front and rear carrier of a Samurai are the same as the front of a Track/Kick,
but I don't know why there has been no front lockers for Track/Kicks until
Trail Tough introduced their front locker, I think it's a rear Samurai that has
some machine work to fit it to a Track/Kick

Wild

The stock Tracker front has a single pin differential, so the Detroit Ez-Locker will not work.  You will need to swap in a Samurai diff using the 5.12 gears.

Wild:  I think your right, but I've never seen one to know what exactly they are doing to the diff to make it work.

I bought and installed one and I couldn't
tell you what they did to it 

Wild

I bought and installed one and I couldn't
tell you what they did to it 

Wild

The carrier is machined down to allow the Kick half shaft to fully seat in the carrier (larger diameter than the sami axles)...

PLUS: you need a carrier PIN retainer sleeve if you are running any thing smaller than the 5.83 aftermarket gears. If you are wanting to do this. Hawk sent em everyting I needed to so the install on my 89.

Zig

PLUS: you need a carrier PIN retainer sleeve if you are running any thing smaller than the 5.83 aftermarket gears. .

Unless you are running 99+ front gears.  Those ring gears are cut differently and hold the pins in just fine....from the ones that I have seen.

Front and rear carrier of a Samurai are the same as the front of a Track/Kick,
but I don't know why there has been no front lockers for Track/Kicks until
Trail Tough introduced their front locker, I think it's a rear Samurai that has
some machine work to fit it to a Track/Kick

Wild

To clarify...
88.5 and up samurai front carriers are identical to the trackick front carriers (single crosspin, split at the ring gear).
Samurai rear carriers and 86-88 samurai front carriers are the same (3 crosspins, endcap comes off).  No trackick carrier ever had this design.
There have always been trackick front lockers...they just weren't marketed as such.  A 1510 locker will fit and work in a trackick front carrier....but you will only be able to use 1 crosspin, instead of 3, that are normally used with a locker.
To use a samurai 3 pin carrier in the front of a trackick, the end of the carrier has to be reamed out so the trackick passengers side shaft will fit inside it.

A samurai

I found a Detroit front locker for a Tracker, locally. Its used, the guy said it fits the front of a Tracker or Samurai. Does a Tracker front locker also fit a Samurai? Also what should I look for when buying a used locker?

It is likely a 1510 type "samurai" locker.  It will fit in the front or rear of a samurai and the front of a trackick (see my post above).  Look for excessive wear on the teeth (where the sidegear teeth engage it).  Make sure nothing else is excessively worn or broke.  Make sure all the parts are there.
This should show you the parts that should come with the locker...
http://www.petroworks.com/drivetrain-differentials.php