However, I'd argue that most people in the business really struggle as they don't understand what makes a good domain name and what's a bad domain name. While I do not work in this field full-time, I can say that I can make a living of it, but it's probably because I don't ask for absurd prices (e.g. randomword247.com domain name for $8000 is just unrealistic and won't happen) and therefore can sell domains easier.
Namejet is a domain clearing house (at least that is my term for it). What happens is that people let their domain names expire everyday. Some people just don’t need their domain anymore, some people forget to renew it, some people die, I mean there can be hundreds of reasons why a domain will not be renewed. What the domain registrars want to do is cash in on the domain that someone forgot to renew, so they offer that domain to Namejet to auction it to the general public. Think of it like a storage unit, someone doesn’t make their payment and the storage company hires an auctioneer to come and auction the storage unit to the general public. Namejet is doing this but for domain names. The bid for each domain starts at $69. What I LOVE about Namejet is the process… they let you search through the domains that are coming up for auction 30 days in advance. Whoever places a $69 bid on a domain will be entered into an auction when the 30 days runs out with everyone else who placed a bid. If you are the only bidder than you get the domain for $69. If there are 10 other bidders than there is a closed auction, meaning only you 10 guys can bid now against each other in a 3 day auction with the highest bidder taking the domain when the clock strikes zero. I love this because let’s say there is a great domain and it flies under the radar. If it goes to auction with only 3 or 5 or 7 bidders my chances of winning this domain at a very low price are great. I’ve won domains for $100 which I was able to flip for $2,000. I won a domain for $2,500 which I flipped within 4 months for $7,500. I won a domain for $1,000 which I’ve turned down $5,000 offers on because I think I can get $10,000 for it. The key is to not overpay in order to leave yourself plenty of room to turn a profit when you flip the domain.
Here's an old post on search engine roundtable that claims google's policy is to discount previous backlink juice when a domain changes ownership. I'm not convinced whether this is actually true or something Google says to discourage excessive domain buying / 301 redirecting for SEO benefit. The comments above seem to give varying opinions on this matter. Would be great to get to the bottom of that one!
There are several types of domain names to avoid. One is intentional misspellings, as these make it less likely someone will search for them. You also want to avoid names with extra symbols like hyphens, or with added prefixes and suffixes like "e" or "my." Outside of a few notable exceptions with very strong brands (like eBay), those additions can only hurt the value.