I'd think the market would be limited to someone wanting a custom SLX frame. Most framebuilders back in the day went to a mix of SL and SP for frame sizes bigger than 58cm because they considered SLX too whippy for larger frames.

To the right buyer, I'd guess the kit is probably worth a couple of hundred bucks, but the best way to find out is to put it on ebay with a set minimum bid that you'd accept. To get the highest price, you'll need good quality photos of all the parts and the packaging included in the listing. You'd also need to measure the length of the down tube so you can specify whether the kit is Compositoin A, B, or C.

With all of the air-hardening high strength OS steel tubes available today, the value of an SLX kit is going to be mostly nostalgia.

My $.02.