Theoretically, it's possible via the find and replace function when you bulk edit in selling manager, but I should mention I've tried to use that function a few times and found it very hit and miss (mostly miss), so I ended up having to check which listings had updated properly, then manually change the ones that hadn't. If you want to give it a try, just select as many listings as you would like to edit, then when you're bulk edit view, click 'edit fields', then select 'item description' in the dropdown menu, then once the window has popped up with a text field, change the action in the dropdown menu to 'edit listings in bulk - find and replace', the edit window will then give you two fields, one to input the current text, and one to input what you want to replace it with.
The business policies (if you have them implemented) can do that with return policy information (in a more limited way, though), and there's a spot for a custom message / instruction that would apply to all listings, however that will only appear in the postage and payments tab, not the listing description, so buyers will only see it if the switch tabs to view more P&H info, or to read the full T&Cs of a return policy.
