If it's a good name and in good condition you could advertise (subject to membership) on www.aqha.uk.com, www.wes-uk.com or free classifieds on www.westernscene-uk.com . At least look on those sites for comparisons.
Have a good hunt around on it for a maker's mark or serial number or anything that could identify where it came from; ideally, people may want to know:
- what type of Western saddle it is - reining, cutting etc, there are differences in the build according to the usage for which it was intended
- tree type (wood, Ralide etc)
- rigging style
- rough weight
- seat size
- bars
Useful also to take photos of side view, seat and underside of the saddle.
With western, Aussie, Portuguese and Spanish saddles there are a lot of Indian imports which are not on good trees and are the equivalent to buying a cheap Indian GP saddle - just not very nice (but trade they are under £100)