It is an very raw idea,but intrdouce some mechanism of solidarity payments to teams which retain own their drafted players from a certian amount of potenial(let's say MVP give or take areas,could be one level below or one level above ball park areas) would be a good incentive for lower leagues teams to retain and train by themselfs talent and avoid extreme "Draft&Sell" situations(they can still do it if they will) scenarios.
Theoreticlly speaking,let's say the player "Mr.Worldwide Meme" from "Buzzerbeater" team drafted in the next season,stayed for around 4 seasons but unfourtanely did not fullied his full celling,sold for another 5 times in his career and from each time they get up to 100,000 per player from solidarity payments(None if he drafted and sold at his rookie season,25,000$ if he sold at his second constructive season,50,000 if he sold at the third year,and the 100,000$ of the fourth season would be payed from buzzerbeater,not the purchasing team).