Firstly, we don't regard these as 'ours' to sell in any way- they are produced as a paid service to a client, the artist, and thus they belong to the client.
Secondly, we strongly support Artist's rights, and their right to copyright - it's something we've taken very seriously from day one here at Image Science. We will never do anything that violates the artist's right to decide and control how their work is released to the market (what happens once it is in the free market is not something we control, of course - although artist's might want to note Australia does now have law regarding secondary sales and is in the process of making international agreements in this area). Releasing proofs - be they signed or unsigned - would be a violation of our implicit ethical agreement with the artist to produce work only on their behalf and under their instruction.
Thus we do not offer artist's proofs (or any other secondary material) for sale. Any uncollected material produced as a proof etc. is properly destroyed and recycled.
Please note we're actually generally against the concept of 'artist proofs' full stop - we believe any print is either a perfect final print, or should not be sold. Unfortunately some artists who are less scrupulous about strict editioning are known to use artists proofs as a way of working around (their own self imposed) edition limits. This muddies the waters of limited edition prints as a concept and really only hampers the artists' own efforts to establish an ethical and well respected artist's practice which is a key part to gaining a professional level reputation in the art world.