From what I've seen the used Model 3's seem to come in waves. You'll see from none to maybe 20 or 40 nation wide for a week or so and then suddenly like 150 a day will show up for two or three days, then nothing again.
From what I was told by a Tesla Sr Sales member is that anything that is on the used page will not qualify for the federal tax credit and is truly "used". Inventory vehicles that have been used as show room demo cars or test drive cars will show up as "new" vehicles but with miles on them, those cars usually can qualify for the fed/state tax breaks.
Used could still be a very good deal if they're seven day returns, but usually more so because it's a model that isn't made any more (like a mid range showing up or Long Range RWD) and only has a few hundred miles on it. Otherwise it tends to not be that good of a deal compared to buying completely new, except that you aren't charged a destination fee... though you will be charged a transport fee unless you are local to it or travel out to pick it up.
Edit: Like I see a blue MR for $42,800. a Blue SR+ would be $41,190 from Tesla. Factor in the $1875 fed credit and the price for the blue SR+ is $39,315. That's $3,485 for almost 10% more range and a full premium interior. While not fantastically priced, you CAN NOT get a premium interior for less than $9k extra right now. Sooooo, if you really have your heart set on all the speakers and sub, streaming music, and a little extra range, less than $3500 seems like a steal compared to what you can do with a new car...