The other poster has the right answers for you.

You will need hard copies if you get audited it's your choice if you want to have it all together in case you get audited to make future you life a bit easier or not.

The only real thing to remember is you can not carry a loss this way so if you sell a $100 item for $50 you can only deduct 50. If what you are doing is complex enough that not carrying those losses matters you should set up a "business".