I've sold a number of handregs, but I always had a reason for registering the name (that it was a .com where the other extensions were taken + it had commercial value, it had keywords that were the same or alternate terms for domains that had recently sold, etc.).
As
@ABDELDJALIL said, just don't overdo it. Handregging is an easy way to spend too much money.
A few examples (and why I registered them):
Bau / Bild / info - "Building and painting" or "Construction photos" in German.
Reasons for registering: It was a while back; I'm guessing that it's because I saw other "bau" names selling in .DE, and .info names were also relatively popular in Germany. I'm not fluent, but German was my language in high school. The combination of the domain being cheap and available, and having keywords that had sales listed in DNJournal, made it a low risk handreg. (Even if it never sold, I was only out $3.17).
Reg price: $3.17
Sold for: $500
Regain / Your / Privacy
Reason: People were sharing a lot on social media, and I anticipated that there would be a backlash. Privacy seemed like the next big thing. I searched — "Protect" (obviously more valuable) and other variations were taken, and the name with "Reclaim" at the beginning was owned by a newspaper in the UK. That was enough for me to see that there was some interest in these keywords. One year later, there was a huge scandal related to privacy, so the timing turned out to be perfect (and lucky).
Reg price: $13.17
Sold for: $5350
There are others — my first sale was a handregged .DE (related to "car rentals" in Australia).
A lot of veteran domainers focus on aftermarket names that have been previously developed, or owned by other investors. It's safer, more liquid, and you stand a greater chance of higher payout. If something has valuable keywords, there's a good chance it's been taken by someone; therefore most of the "available" names that are out there aren't worth very much.
Handregs can be a gamble, but they're cheap. Just stick to terms that have a proven track record of selling, and more than a handful of potential buyers in Google. Good luck!