It's just the beginning. Taking into consideration the volume of new sales, soon there should be plenty of offers.
The cars on offer are typically the Long Range versions from 2017 or 2018 with prices starting above $40,000.
Tesla is selling the used Model 3 with a warranty similar to Model S/Model X:
"Every used Tesla vehicle has passed a 70‑point inspection and comes with either a 4‑year or 50,000 mile warranty or a 2‑year, up to 100,000 mile warranty"
Those interested in a used Tesla need to consider whether it's worth to go with the particular car compared to both - new (even lower-trim version) or used sold directly by owners.
The other thing is that it seems that Tesla is selling customers cars, not waiting on the potential availability of the full-self driving feature. Earlier Tesla decided to lease Model 3, but without an option to purchase at lease end, expecting that in a few years the cars maybe will get a 2nd life as robo taxis.