I have found Art Grab today, a website where artists can sell unique pieces of artwork, e.g. for album covers. After creating a profile I have added a few square designs. The service does not charge upfront fees and only keeps 20% of earnings when you sell something. It think is a fair deal and I will keep you posted about any updates.
If you want to sign up, you may use my referral link: https://artgrab.co/sell?referral=stine1
Or check out my profile here: https://artgrab.co/art/stine1