There are currently two main types of domain speculators: those that buy domains, build sites around them, and then flip the domain and accompanying website, and then there are those that buy and sell domain names without web sites attached. While both can be very lucrative businesses, the second type is much easier for novices to learn, and as such, shall be the topic of discussion for this article. 
The backlink profile of a recently-available domain is a double-edged sword. While some expired domains have been well cared for by their previous owners and have high-quality links from reputable websites, some contain quite questionable backlinks. The worst case scenario is that the domain was merely used as a link farm to strengthen a major project in a satellite domain network and has already been penalized by some search engines.
There’s an important distinction to make here between domain flipping and website flipping. The latter mainly refers to buying and selling full websites. By full websites, I mean websites that actually have content and more often than not have traffic and revenue. When buying and selling websites, the domain name matters less as the main value there is the content, traffic, revenue, history/reputation, sustainability and growth opportunities. The same can’t be said for domain names where you’re just selling “the name” and hence it’s all that really matters.