Kato Kaelin didn't even have a proper suit when he testified in the O.J. Simpson trial ... and the guy who loaned him one is now about to cash in.
Turns out Kato borrowed the olive green Gianni Versace suit from a friend when he testified in 1994 -- it's been unworn since, hanging in a garment bag.
The owner of the size 40 threads is asking for $5,000 on eBay ... and even includes a letter of authenticity.
Gloves sold separately.