If you happen to
like PDF, then ebooks.com is one of the few sellers left that carry them. I still buy books from them that I want in PDF (I've yet to see an epub or mobi of something like
this book that isn't a complete horror show).
Also, books that are available in both epub and PDF can be downloaded in both formats with the same purchase. If multiple formats are listed as available, you can download all listed formats. Note, however, that books are downloadable in whatever the currently available format is. HarperCollins, for example, used to offer most of their books as PDF and epub, but the PDF option has been removed from a number of my ebooks.com books as HarperCollins stops authorizing their sale. On the other hand, I've had formats
added to my books, too.
If any of your books allow multiple format downloads, you'll see multiple options in the download dropdown. I've attached a screen grab so you can see what you're looking for. If multiple formats are listed, then you can download each format separately.
The biggest downside to ebooks.com is that they do
not discount prices. Their prices will sometimes reflect a publisher sale, but they usually are straight retail price.