There are several types of domain names to avoid. One is intentional misspellings, as these make it less likely someone will search for them. You also want to avoid names with extra symbols like hyphens, or with added prefixes and suffixes like "e" or "my." Outside of a few notable exceptions with very strong brands (like eBay), those additions can only hurt the value.
Your domain name registrar may provide you with a free one-page Website tool, which you can use to create a “this domain is for sale” landing page. Alternatively, you can create a single page (perhaps a hidden page hanging off one of your existing websites) that indicates that your domain names are for sale. You can then forward all of your domain names that are for sale to that one page.
Most domains traders, or resellers, buy hundreds of domain names. This way, the chances of hitting the jackpot are higher. But, this takes some up front money. You can start with a handful of names and work your way up. This is the best way to begin. You may also decide to purchase a name from another seller if you think that a name will be big enough.
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.