I would think the percentage of people who shop online but don't have a printer would be pretty small, but if you happened to stike one you would probably have to ask Ebay for options.  If you are worried about striking one AFTER you have printed the label, surely in your discussion of the return issue with the buyer you could ask them if they have a printer and explain that you will be sending a prepaid label which they will need to print? If they don't have a printer, then you could send them a prepaid satchel by mail perhaps?