In my last post, I noted that my copy of Ernest Hemingway's For Whom the Bell Tolls is not a first edition — as defined by collectors. It is, however, a first edition as defined by publishers. Collectors (like me) tend to use first printing and first edition interchangeably. Different claims from ebay sellers add to the confusion. Bookseller Craig Stark explains the differences in his 2003 article on the BookThink blog, Basic Edition Identification.
It appears my copy is a first edition because it was printed with the original plates; no changes in content of the book. But it was not the first printing. So, is it more valuable than I thought? No. The value comes from what collectors will pay for the edition, and in this case the first printing (or "first state") is way more desirable.