Well, unless you got super powerful domains with 3 letters or numbers or something, you're going to have to work hard to build an inventory and list of buyers you can market to. Well, that's the way with any online business. It ain't easy.
Correct, you need to be scraping through popular trends and find domains related to the trend to be able to get your hands on some of the best domains. I agree that 3 letter domains or even an English word as a domain is something that would have potential to be of interest for a buyer.
The other important thing is to market it at the correct place for sale. If it is marketed at the wrong place for sale, then you are not attracting the right customers.