Here are my thoughts:
1) The best time to sell your draftee is THE MOMENT THE DRAFT HAPPENS. The first few pages of 18yos get a tonnnn of views on the TL and there is a veritable feeding frenzy. If you miss out on that ~half-hour chance, you should wait for a high usercount time (europe gametime, US rush hours, whatever) because the main thing you want is two people to say "THAT'S my guy!" and then fight over him.
2) It's rarely worth it to wait on selling your draftees. If you get a guy with a suspiciously-high salary, you might try training him a week in his primary skill because a double-pop to strongs does draw some attention, but that's a risk and I'm not even sure it pays off in the best-case scenario.
Mainly though, there's pretty much an instant feeding frenzy and people drop unreasonable amounts of money the first couple days. Then it dies down until near the allstar break, at which time the supply is so low that you can often get up to $300k or so off new accounts that need youth and have starting money.
As far as selling trained players, the peak value these days seems to be around age 23-24 in terms of sheer net income. Well-trained reasonable-salary guys that age can go for many millions. It seems to me that the hive mind draws the "young player" line after 24, and it's hard to keep salary in check training past that anyway.
However, I think in terms of profit per training week, the smartest time to sell is 19 or 20 years old, because of teams like me. My league is too competitive for 18 year olds to ever take the court intending to win, but I still want a young player that I can mold to my needs. I've been looking for weeks for a 19 or 20 year old big with good ID and PA that I can round out how I want while still winning some league games. You can charge a little bit of a premium for those guys because the supply is low but the demand is fairly high -- an "ID-only" 20 year old goes for ~800k, which is comparable to what a similar but older and tri-skill C was going for last I checked. So all those guys that spent three seasons training up IS and RB didn't actually make any extra money off of it. I would have paid the same amount when he was 20 so I could be free to do what I want.
If you're setting out to do something like this intentionally, definitely do it with ID or OD. Every player needs defense. You could make a killing like that I think
