I use to work at one as a pricer (15 years ago not a goodwill though). It's not necessarily getting harder to find deals because of more competition on the sales floor. Most the good stuff never gets there. There are businesses and collectors that get items sorted out before they have a chance to hit the floor. The more retro,vintage,antique stores that go up, the less of that stuff hits the floor.
For example we'd hold musical instruments for one buyer, furniture for a few different people and stores, the appliances were sent to the rehab center our store supported to train the patients "job skills", tools for another place....etc These folks would come in and buy most the lot we held for them. Their rejects then went to the floor.
You gotta remember, these places exist not to save you money, not to recycle, but to make money. And the back room sales make the most sense business wise. We pretty much assumed that 80% of the stuff that hit the floor was going to the dumpster soon anyway.
Though even though I was an art major in college with a minor in art history. I probably wouldn't have thought that the Picasso was real, You got about 5 seconds to price everything, or to pass it on to who ever knew more about that type of item than you did. Though if it was a print of his work from certain periods of his career, it probably wouldn't have hit the floor, but would have been hanging on my wall.