Buy/sell where you have unique expertise. *If you work in the toilet hardware industry, buy/sell stocks related to that industry.*
I have yet to meet/ hear about anyone who has been able to do this successfully. I personally have tried this with my own segment of industry domain, not that successfully. The issue was tunnel vision that comes with knowing too much about your own segment or falling into “insider trading” zone.
Most of the success came from adjacent segments. You have enough knowledge and experience to make an educated guess on the adjacent segments but you are not involved enough to be knee deep into it and can’t see the forest from the trees.
I don't claim to be any kind of investment guru, but here's my two cents.
I've had solid returns from investments in industry segments I've worked in. The strategy outlined above by wegs2 reflects my thoughts almost exactly, including not investing more than 25% into these "personal experience" investments, since it inevitably means putting many eggs in few baskets. Most of your investments should be in lower-risk, highly diversified things like index funds.
I'd add a couple of additional things:
1. I'm pretty confident about predicting the success of an industry, but not individual companies. There's just too many factors in play to predict individual winners and losers. So within the "personal experience" investments, I hedge across several competitors who I think have good prospects. (Happily, all such companies have done well, so I guess I've avoided winner-takes-all outcomes.)
2. Even in industry segments I'm very familiar with, I avoid companies that are prominent household names. I think there's just too much investor psychology wrapped up in this, and overvaluations are more likely. So I avoid investing in companies like Google and Apple. (Granted, if I had invested in Apple it would have done well! But it being a household name gives me cold feet.) One way to avoid this is to consider upstream suppliers the general public may not be familiar with, but nonetheless produce great value and have good prospects.
I limit it to products I consume but am an early adopter on. It has worked great on TSLA and ENPH. I got into BCLI because of Making a Murderer 2 attorney on Twitter talking about an ALS treatment her law partner’s wife was taking. Big fan of index plus 2-5 stocks.
Most of the success came from adjacent segments. You have enough knowledge and experience to make an educated guess on the adjacent segments but you are not involved enough to be knee deep into it and can’t see the forest from the trees.