I've used the
www.estatesales.net site quite a bit as well. I haven't scored anything big sports-related. The only baseball stuff I come across are 80s/90s wax junk or late 70s. I remember one sale had 4 shoe boxes of cards - mostly baseball, but some old basketball - all from mid-to-late 70s. The cards were well loved and had a few stars like Steve Garvey and Pete Rose. I can remember a few people trying to grab the boxes and they ended up fighting over them, which started a price war with the cashier. In the end, one guy paid $300+ for 3 shoe boxes of very worn cards.
On another note, not baseball related, I did strike a great deal on two Canon lenses for my digital SLR. Retail the two lenses were $4k+ and I paid $900 for both (no haggle - that was the listed price).