With domains, antiques, car number plates and other near unique items - to make a return you need a regular margin. This comes from buying in one market, and selling in another, or adding value in some way.
Domain names are tricky, it can be hard to find ones offering a good return, and turnover is slow at end user prices. There are no established venues for regular buying and no reliable way to generate consistent sales.
Relatively few end users understand the merits of a good domain, at the prices they command. The recent flood of new extensions has diluted the market.
Is there scope to earn a living as a full time domain seller starting now? It really depends on experience, resources and income requirements. I think anyone with the ability to succeed would achieve a better return by focusing their efforts on related services, rather than just domain sales.
A sales platform with set prices will provide turnover, but not the best margins. It does however provide a steady stream of customers, and other opportunities.
The popular model for domain sales is a large portfolio, and parking pages with a "make an offer" link. With enough capital and quality domains this plan works, but it relies on careful buying decisions and reaching a viable size. Nokta have taken this to the extreme.