Advice from several years in the industry...
-most individuals lose money their first 2-3 years in this industry and very few individuals actually make a full-time living merely from buying and selling domain names
-selling domains is not easy, rather it is far more difficult than one would think by looking at sales reports
-there are far far more domains already for sale on the major aftermarkets - SEDO, Godaddy, Afternic, etc than end users are buying. Don't be delusional and believe that you can just register a bunch of domains and the cash register will start ringing. Most anything you pick up probably will not sell in a short timeframe.
-the few domains you are able to sell will have to pay for the majority you do not so focus on quality (not quantity) as one good domain cannot carry the weight of hundreds of domains no one wants
-resist the temptation to hand register hundreds of domain names which are available. If a domain is still available to register after 20+ years of the internet's existence, it is likely not very valuable.
-resist the temptation to acquire domains in any extension other than .COM until you have learned the type of domain that end users pay money for - short, memorable, commercial use and in most cases .COM
-if you acquire domains other than .COM make sure they are a small portion of your portfolio. .COM is what sells not all the obscure extensions that have recently launched and which novice investors are wasting precious resources on thinking they have the next best thing to Travel.com History has shown repeatedly that new extensions turn out to be poor investments for most participants.
-learn about Google's Keyword Planner tool