On a less formal note, I recently went looking for books on eBay and found the site inundated with what appeared to be Project Gutenberg and other texts from public domain sites reformatted and offered for sale by print-on-demand publishers for $40-50 apiece. Scholarly books are also offered for sale in print-on-demand version, many for two to three times what the book would cost used. (For example, Michael Anesko's Letters, Fictions, Lives: Henry James and William Dean Howells is $128 in a print-on-demand version on eBay and Amazon.com and $36 from used booksellers on abebooks.com.)
The "caveat emptor" part of the message is this:
(1) If you need a scholarly book, WorldCat can tell you if a nearby library has it. (You can also find a WorldCat search box on my home page at http://www.wsu.edu/~campbelld/index.html and can add one to your own page or weblog, if you want to; it's free.)
(2) Unless you are eager to pay two to three times what the used booksellers charge for a scholarly book, try them first. You can still find original editions and used books on eBay, too, but since the big online stores don't push them, you may have to scroll through several pages of print-on-demand repackaging to find them.
(3) If you're reading primary texts for enjoyment and don't mind reading texts online, check the various books online sites first, including Google books.
(4) The old editions of a less-famous author (like W. D. Howells) are often more reasonably priced than the print-on-demand versions.
(5) With print-on-demand versions of primary texts, it's not always clear what edition you're buying.