In the past, expired domains were used as part of black hat SEO in order to generate backlink sources quickly and easily, and this practice is still used somewhat today. Getting links in this way is not a recommendable practice. All relevant search engines work nowadays with highly complex ranking algorithms, which can no longer be outsmarted by these link networks.
If your domain name is related to a particular niche, then you may find success by advertising in forums that are related to that niche. Simply placing a link in the signature of your posts can tremendously speed up the selling process. While it may be possible to find prospective webmasters on niche-specific forums, it is generally better to advertise on webmaster forums, where there is a high volume of potential customers.
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.