Erin said that we will see this feature soon. Not in 3.0 but maybe in 3.1
The absolute earliest 3.0 could launch is Ciizencon, but I think a more reasonable estimate is that they push hard and delay it to early next year.
They then need to get 3.0 to a somewhat stable place, not crashing all the time. You can't have people working hard for ships then suddenly they disappear, or players crashing / disconnecting and then someone comes along and steals their ship. They'll also need a variety of missions working reasonably well as that's the only way to earn money right now.
I suspect all of that will easily take all of 2018. By early to mid 2019, I think we could see an early release of 3.1 with mining. The code base seems so glitchy I don't think they'd try implementing interactable objects prior to that. Given how particular Chris Roberts is, I could easily see 3.1 requiring even more time, so that the asteroids break apart in realistic ways after superheating them with a laser. Probably need to get them glowing just the right away as well, etc. So looking at mid-2019 before mining is actually working decently well.
If they add ship sales at that point, I'd consider them "ahead". But frankly I think Chris knows how his bread is buttered and will save this feature until after Citizencon 2019. If you add in-game ship sales too quickly, players will have far less incentive to purchase the newest ships, and by 2019, CIG may be running low on funding or may have already turned to external investment (if he hasn't already).
All of this is also under the assumption that they don't make any progress with SQ42, which is almost certainly going to take time away from SC. So probably my estimates are too aggressive, it could easily be 2020 or 2021 before we see in-game ship sales.