They could be very useful... but why did you post them HERE if we have to pay to download? Excuse me, but I think it isn't correct. We must pay? You must pay for some advertisement instead of publishing them here!!
It's not unfair because deviantART has decided to offer most of their services for free. They don't have any inconvenience with that, since there is no rule that says you cannot advertise your work in this website. In fact, it is a very powerful tool when it comes to advertising, not even once has deviantART told their artists to stop selling commissions just because they don't profit out of them, because they know they didn't do the work and it's fair for artists to gain money for their work c: Besides, no one is forcing you to pay for them, if you don't agree with them being sold, just don't buy them. Your unwillingness to spend money on this does not automatically mean it's unfair c: