I'm a rookie in this business but let me express my first impression.
First, a staker must have solid bankroll management too. Those with bankrolls in the low 4 figures can't afford to buy 2-digit percentages in the majors, especially because most of the ROI is eaten by the markup.
Imho buying shares is for those stakers who yet lack a roll or don't wish to work on it full-time, or for big seasonal series like COOPs, FTOPS and Poker Maximus when fields are softer than usual and thus staking volume increases. It's the simplest, but simple jobs yield low income, don't they?
The problem is that players who sell shares usually don't want deep and longterm relationships with investors, giving them only OPR or Sharkscope stats, which might lie about their ability, especially if the sample size is small, and it often is because MTT stats converge slowly; and surely players hide their shortcomings well. It's hard to weed out regfishes who have accidentally hit some big final tables and think they have a lifetime license for selling action.
That also influences investors' BRM. Compare them to players who select tables and who don't. Those who sit at just random (or Zoom) tables should have a stricter BRM as their winrates will be lower. Those who neither 'bumhunt' in the Marketplace nor review horses' applications thoroughly but instead buy everything should have a stricter BRM too.
There are both overpriced players who sell packages quickly because they have an army of loyal, often irrational, investors (friends) and those who are yet unnoticed but of good value (mainly due to the small sample size) and deserve investing double-digit %-ages. It's hard to find the latter (without digging into their thought processes and grinding routines). I think I can't do this well.
The value of a player is determined by both the markup and the estimated ROI (Bayesian inference is needed here). A consistently cashing player with 35% estimated ROI selling at 1.1 is undoubtedly better than one with 45% ROI derived from a single final table, selling at 1.25 and stuffing packages with side games where he's a fish or has a lol sample size.
Those who hand out longterm MTT stakes supervise players much closer, know a lot about their thought processes, mental game, even real life events, so they can assess the players' ROI preciser and also hire the right coaches and plug the most important leaks for mutual benefit.
As for big scores, look for them in WCOOP event packages, of course tons of regs there are staked (because the events are softer than usual and the regs feel like taking a shot). For example, a 2+2'er MauriceSch, who has just won the $10300 WCOOP-23 for $533.2K,
said he had sold 80% of his action (not in the MP), so if someone had bought 5% of it (worth ~$500), this share has returned >$25K.