reproducing (for sale) artwork that includes the use of postage stamps

Most government work is not subject to copyright but stamps are. There is a licensing division of the US Postal Service that deals with this specifically. I have attached a link to their website. It appears you are looking to commence a significant enterprise using stamps, so you should sit down with an intellectual property lawyer in your area and not try to do this yourself.

US postage stamps often display paintings and photographs that are protected under copyright law. Unless those paintings or photographs are in the public domain, they are licensed to the US Post Office for stamp use by the owner of the copyright. Moreover, the US Post Office claims copyright in the stamps themselves. The rule you need to be aware of is that only the owner of the copyright in the painting or photograph or the US Post Office can use US postage stamps to create a "derivative work" that includes the stamps. Collages are derivative works.

I suggest that you review the US Post Office website information regarding their rules for seeking permission to use stamps in derivative works.

http://www.usps.com/rightsandpermissions/common-use-trademarks-copyrights.htm
http://www.usps.com/rightsandpermissions/stamp-use-information.htm

While I do not endorse the information, the information at http://www.funnystrange.com/copyright/ provides some useful background guidance on collage copyright issues.

In short, you are quite right to be concerned that offering collages for sale that contain partial images of postage stamps raise copyright issues. This is a tricky issue. You should seek legal assistance from a competent copyright attorney before offering for sale any of your collages.

While the above answers are correct for any reproductions of the collages, they are incomplete for the collages themselves. Copyright only protects making a copy of a work. It does not protect against buying a legal copy of the work - in your case, buying stamps from a post office or receiving them in on something sent through the mail - and then selling or doing anything else you want to the copy you legal obtained.

Assuming the stamps you are using to create the collages are not stolen or illegal reproductions, you are free to use those stamps any way you wish, including making a collage out of them. This is the first sale doctrine, which states that once a patent or copyright owner sells a tangible good embodying the work (for copyrights) or the invention (for patents), the owner has exhausted its rights in that tangible good sold, and the purchaser is free to do anything he wants with it (with a few exceptions that aren't relevant here).

So, if you were to sell the collage itself, you would be selling the actual physical goods - the stamps - that were previously sold by the copyright owner, or by someone who themselves bought from the copyright owner, not a copy, and thus would not be violating the copyright. That is not to say that you have to sell them for the same price you bought them for - you can charge whatever the market will bear for turning them into art.

I respectfully disagree with Mr. Roe. The "first sale doctrine" of which he speaks (codified at 17 USC 109(a)) expressly limits only the distribution right afforded to copyright owners and not any other right within the copyright bundle of rights -- such as the exclusive rights of reproduction, display, and to prepare derivative works. Precious Moments, Inc. v. La Infantil, Inc., 971 F.Supp. 66, 67-68 (D.Puerto Rico,1997).

In the states within the 9th Circuit (consisting of most of the west coast) it's a general rule that while the owner of a copyrighted work may sell or otherwise dispose of the work, he or she is not free to use that work to create a derivative work by using the original. Mirage Editions, Inc. v. Albuquerque A.R.T. Co., 856 F.2d 1341, 1344 (9th Cir.1988). This rule is not well-received elsewhere in the country -- including in the Texas federal courts -- but if your intent is to sell your works nationwide then you may likely be subject to the jurisdiction of one or more of the federal courts in the 9th Circuit which are required to follow the 9th Circuit rule.

In short, though your question inquired into the copyright implications of reproducing and then selling your collages (which the first sale doctrine unquestionably does not immunize) even the sale of the original collage may be an infringement of the postage stamp copyright.