In my limited experience of shopping in used record stores, each store has a blind spot or two and tends to charge less for certain categories than you would pay in other used record stores or on Discogs. This is always true of thrift stores, junk stores, and other outlets. You develop a feel for what may be good (and underappreciated).
The main blind spot categories (again, in my limited experience) are obscure 50's rock / r&b; country western; obscure vintage jazz (from before the 60's--any of the big names and labels, e.g. Blue Note, are likely to fetch a premium price; 60's-70's jazz; mid-century instrumentals (what is often lumped with easy listening); black gospel (very hard to find in good condition); vocal music from the 50's-70's; world music (e.g. Afro-Cuban); experimental / offbeat music. Some categories like classical and folk are hard to pinpoint. There are some records that were so popular in their day that if you can separate the wheat from the chaff, you can find some very cheap bargains anywhere (e.g. Henry Mancini--keep an eye out for "Mr. Lucky Goes Latin").
Sometimes while the book value (whether Goldmine or Discogs) for a given piece may not be that high, the quality can be excellent. Example: this budget label reissue of "Way Out Wardell" that appeared a year after the original release, and is now selling on Discogs for $12 in Near Mint. Shamefully, I had never heard of this wonderful tenor saxophonist when I found a copy (probably in VG or VG+ at a roadside flea market for a buck or two. Again the rewards of such bargains are more in the discovery than the book value. Trust your gut feelings; at worst, you're only out a few dollars; at best, you've enriched your listening experience immeasurably.
