Unfortunately, most domain names take at least a couple of months to sell, especially without an attached website. Thus, there is no need to become discouraged if your domain name does not sell immediately. Most people make the mistake of quitting their domain speculation endeavor after waiting for several months to sell without success. In reality, domain name squatting can be just as profitable, and it is a valid form of long term investing.

Acquiring what you believe may be a valuable domain name is the easiest part of the equation.  Finding the right  buyer is much more difficult. Using auction sites and forums is perhaps the best way to find qualified buyers that are interested in your domain, and using auctions will take some of the stress away from establishing the value of a domain name yourself.
ExpiredDomains.net gathers all the information you need to find good Expired Domains that are Pending Delete and you can Backorder. Depending on the domain extension you can search through thousands of domains every day before they get released to the public and pick what you like. ExpiredDomains.net currently supports 449 TLDs. From the classic gTLDs like .com, .net, .org to Droplists for ccTLDs you can only find here and now we even support some of the best new gTLDs like .xyz and .club.

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. 
Aside from places where you buy and sell domain names, there are some companies around that support the business of domain flipping. GoDaddy is one of the bigger names out there that does. There, you can not only trade domain names but also park those you’ve bought. The buying, parking and selling is relatively painless and all you must give up is a small percentage of your selling price.
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.
Length: This is pretty self-explanatory. Generally the shorter the domain name, the more valuable it is. This difference in value decreases exponentially as the name gets longer, though. For instance, the difference in price between 3 and 4 letter domains are significant, while the price difference between a 7 and 8 letter domain would usually be much less significant.