I will say editing photos in eBay on my phone (which K take photos) is annoying. Many times it keep defaulting back to the original photo. And I’d you’re posting the same listen by on Mercari you’re doing it twice.
What I do now is
(1) edit it before uploading to eBay,
(2) create/post the eBay listing
(3) open up another window for Mercari
(4) upload the same photos as eBay
(5) copy/paste the eBay listing title into the mercari listing
(6) copy/paste the eBay description into the mercari listing
(7) post the mercari listing.
I can probably create both in under 2 minutes once I have the photos ready to upload….if I was in a rush.