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.
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.