Maybe there's a coupe of issues here Pete.Firstly not everyone has the knowledge to find out previous prices/value,or can even be arsed.They may have friends etc, who's opinion they value and take regarding pricing,which may have nothing to do with Price guides.
Secondly as Ernie Andrews points out ,,there's popularity atm.Not 5-10-15- etc years ago,but now.Might not agree with it, but prices on some records are dictated by that. Imo,pricing is a nightmare...if you sell something too cheap folk think you don't know what you're doing - ie: mug.!.
Plus,when times are hard,,and money's tight..........
Grading tho'.....different ball game.Fairly easy to learn ,if you're honest and want repeat sales.?
If you're happy with the deal you've done ,wether over or under priced is all that matters.