Do any UK based artists or craft makers use these with online shops to sell their work?
I am a photographer and i have not used any of the sites above, but one thing I did was read the Terms and Conditions very carefully to ensure that I retain copyright to my works. There are some sights out there who actually reserve he right to do with your art work as they please, not just for promotional reasons but also to sell on in a roundabout way. There are many photographs sites out there who’s practices are less then honourable to the artist, so be very careful. Also bear in mind that some of the companies you are using are American and therefore US copyright laws exist, which is very different form that in the UK. There you have to register with the Copyright Agency your work or it ma not be fully protected. Also check out that your designs can not be easily copied from the website, as otherwise you will find your designs available cheaply on Ebay.
