I wouldn't worry much about that. Goodwill, Salvation Army and every other mom & pop thrift store has access to the Internet, eBay and every other research tool that we have and I guarantee that they use them. I can't tell you how many times I've walked into a thrift store, walked up to an item to see a printout from eBay or Amazon taped to it to show me how much it's worth when in reality they are only asking prices.
I went on a few gold/collectibles buying trips with a local dealer last year and these two 80ish year old women came in with a few things for us to look at. No less than 3 times they referenced eBay's completed auctions and what the item is worth or what it has sold for. My point is that as time goes on, more and more people are becoming eBay conscious and research items before putting a price tag on them and thrift store owners are first in line. Yes, I know there are still deals to be found and there will always be but it's a new game now and time has changed things.
Personally, I was fortunate enough to pull nearly 5,000 near mint, rare, & promo classic rock albums from our local Goodwill & Salvation Army stores over the course of 3 years or so for the whopping price of .50 each. When CD's started getting big, many of the radio stations were bringing in boxes and boxes of awesome albums for donation and I was hauling out boxes and boxes of the same albums to build my super sweet collection.

.... then 15 years later my apt flooded and I lost 90% of them