This is the advice on the LCSCA site
http://www.lcsmith.org/faq/worth.html
Forum posters are asked to review the FAQ before asking for a valuation, but seem not to. It does get old.

Several here contribute when able on the Shotgunworld 'I.D. and Value' forum, including Researcher
http://www.shotgunworld.com/bbs/viewforum.php?f=5