What is a durable and affordable flooring covering for a shed?
Look at a flooring store for sheet vinyl also called linoleum, remnants (the end of rolls). They can be inexpensive and would be seamless. An alternative that would be more slip resistant is something we call locally  porchpaint. It covers many New england porches. It is only available in gray and is an oil based enamal. You can spread sand on it while wet to increase traction. I have had it last 10 years on a porch floor.