I personally don’t think that you should in all good conscience sell any climbing gear that is that old, and certainly not the software. Most manufacturers suggest that 4 to 6 years is the safe age for software even if never used and stored in a dry, cool, dark place.
The hardware will probably be OK, but I’d guess that most climbers would steer clear of it at that age anyway so only beginners would buy it, and they probably aren’t best placed to make an informed opinion of its safeness.
Ask on http://www.ukclimbing.co.uk for more opinions, but I think that 15 years old is too old to be safe. Plas y Brenin did some tests on ropes a lot newer than 15 years with some scary results (broke after 1 fall rather than the specified 7 or 8).
Probably not what you wanted to hear, sorry.