Yes, this is legal under the doctrine of first sale. In general, under the law, distribution is an exclusive right of the copyright holder. However, this is a specific exception, where "the owner of a particular copy [...] is entitled, without the authority of the copyright owner, to sell or otherwise dispose of the possession of that copy".

So, you are in the clear reselling photographic imprints you've purchased — including making a business of it.

However, I think your last idea (that your framing creates a new piece of art) is actually possibly problematic, because that's fundamentally a derivative work — another concept in copyright where the copyright holder has an exclusive right. The Wikipedia article covers a case where mounting artwork purchased on notecards onto ceramic tiles and selling the result was not found to be permitted under the doctrine of first sale — the exclusive right to create derivatives was found to be more important. But this would only apply if the result really is a derivative — I don't think normal framing is typically considered to create one.