Polaroid applied for a design mark apparently covering the shape of their prints, but that application was abandoned in 2000. That shape is distinctive, but, for example, Fujifilm Instax looks similar — not to mention Impossible Project film, which is actually made in a former Polaroid factory.
You're not using the Polaroid name, which does have a valid trademark, currently held by a licensing company (since the original is defunct — all that remains is the intellectual property rights). That company also claims the "Polaroid Classic Border Logo" as an unregistered trademark — but as the other commercial examples show, that's not particularly strong. Generally, ™ covers "commercial misrepresentation of source or origins of goods" — if you are using this shape to imply that your photographs are made or licensed by the company with rights to the Polaroid name, then you might be in trouble. But, it doesn't seem that you are.
If you were selling actual prints made on Polaroid film, I don't think the question would come up — everyone understands that art made in some medium is made by the artist, not by the company that sells the medium itself. (Just as you can sell art made of Lego.) In this case, you're not doing that; you are just using a distinctive shape often associated with instant prints. If you're really worried, consult a lawyer, but in my lay opinion, I think you're well in the clear, because you're showing and selling your work, not a competing film product. As long as you don't do anything to imply sponsorship or endorsement, I wouldn't be concerned.
It's probably worth noting that this site's logo uses a similar shape. Of course, Stack Exchange isn't selling photographs or film -- but also, I've looked at that logo thousands of times, and never once did it even cross my mind to draw a connection with the Polaroid brand.