Many have replied already but I would like to stress that unless you have something really premium, like a common dictionary single word ..com, a domain name may not sell for many years and indeed the majority of domain names held by domain investors will
never sell
.
One can compute an average using how many domains are for sale and how many sell each year (while applying a correction for how many are not listed on NameBio). You can twist the numbers a bit but most suggest something of order of 1/100 chance of any particular domain name selling in one year, or looked at another way something of order 100 years to sell (but really it is not like that, many will never sell, some much faster, so it averages that). It is no wonder that people like TheDomainKing talk about domaining as being inter-generational and you will pass on your domains to the next generation!
You can increase probability of sale by choosing domain names with many likely end users, promoting them effectively, pricing them correctly, etc. It is still a long term game if you plan to sell domain names for much more than you bought them for (and you need to to make up for those that would never sell).
Domain investing is not easy, fast or assured, and for those that quickly need returns it is not the thing to be in.
Bob