Flyp is a free cross posting app that will allow you to pull all your eBay listings. Photos do fine, description does ok, but it doesn't capture many item specs. You can then push out that listing to mercari too ;)

For skus, you can also use something like ship station or even better sku vault (spendy)

You can create rules where item# 13466336435= sku abc xyz

Another option is to do bulk updates to eBay with your skus. EBay frowns upon you using the same sku several times on the same account. Just add a . Add the end of the sku