Hi Bleed Green
Domains are commodities just like anything else, and as long as the commodity is a good one that is wanted by others then you can make a reasonable living out of the industry by acquiring/buying and then selling or leasing domains. The best domains of course being single dictionary words or generic industry words/phrases. Forget the numbers in domains or other items.
The trick when buying to sell, is simply to match up your domain you have for sale with a qualified buyer. Listing a domain on the many "sale/auction" sites out their rarely gains any serious ROI on an acquisition. To make a profit from domains, you need to research your potential buyers/industry that matches that domain and then target them with information about your offer (without being spammy). If you can convince them that the domain would be beneficial for their industry/business over and above what their current marketing methods acheive (eg - single word domain that perhaps matches one of their primary sale products that could be easily SEO'd to acheive page 1 natural rankings on Search Engines for that keyword/domain name), then of course you can negotiate a sale or lease price for the domain, that is a win win situation for all parties, as the buyers will gain an ongoing increase in sales that gives them a profitable return on their purchase/lease costs).
The other side of the coin is acquiring domains that are beneficial for infotainment sites that you can setup yourself (or directory sites if the domain is generic enough). This method creates income from other sources, such as google adsense and banner advertising/newsletter advertising to subscribers, etc. A good infotainment site can earn tens of thousands of dollars per month in tertiary income utilising this method. A good infotainment site gains permanent return traffic that in turn generates the tertiary income, without ever selling anything on that website. Pure information and entertainment is all that gets supplied.
Hope this helps