I think the term for this is "market depth". https://bitcoincharts.com/markets/bitstampUSD_depth.html The 3rd chart (and the 4th which is the same thing but zoomed in) shows: if I'm willing to accept as little as X USD / Y BTC exchange rate, how much BTC can I sell? The answer is the area under the curve above the price you're accepting.
If you have a lot more BTC to sell than could comfortably move on exchanges, auctions are the way to go. Sometimes the feds seize large amounts of BTC and auction them off in large blocks, and they not only sell but they go for above then-current exchange rates.
If you have a lot more BTC to sell than could comfortably move on exchanges, auctions are the way to go. Sometimes the feds seize large amounts of BTC and auction them off in large blocks, and they not only sell but they go for above then-current exchange rates.