If you only start using a seller's books, one of the many factors that has to be considered is whether to focus on fiction or fiction. In some cases, this may come to a pre-conference and what file you already have, if any. My personal will would tend to lean towards fiction, but this is only because I'm drawn to certain authors. But there is also strong reason for checking non-fiction books, as described below.
Before we go further, it might be worth considering that these books appear on the list of 100 most valuable books, it's fair to say that the majority is fiction. Less than 10% are in the novel, with the highest rank, Seven Pillars of Wisdom, by TE Lawrence in number 3 and not much more under it.
Having said that, books that are not fictional are very good in certain subjects, even if they do not offer high prices for fiction. In some cases, some non-fictional titles can perform many fiction titles (solely on the basis of demand) and if you focus on specific areas, you are able to make a good income.
Usually make smaller sizes, not fiction, better than big ones. There are also a lot of printing presses that specialize in certain content, rather small compared to fiction publishers who can handle large printouts, so it's without saying that the books they produce can be very scarce. As a result, non-fiction books are very hard to find, which, of course, increases their purchase.
With fiction books, it is very often the author who is worth checking out, although this is of course not a fast rule (it makes buying and selling books so exciting and potentially profitable).
Fiction hardbacks are much more likely to sell than paperbacks, with just a few exceptions, and in detail, the first edition, the first print is absolute will be. Some later versions or printers of certain titles are also valuable, but it is not within the scope of this article to intervene in this topic. Signed books are bonuses.
As with anything else in business, research implementation can help you automatically so you should always be ready to study the market. Watch new feeds and new potential top sellers for you, hunt them down and get them for the lowest price you can, then sell them for the benefit. For books that have been around for a while, find out if you can actually sell which books and you'll be running and setting ahead of other new sellers.