There does seem to be an issue with contacting sellers if you use the app.
However, you don't need to contact the seller.
Start a return and you will be able to message the seller through the opened case.
To request a return, select the item you want to send back from your recent purchases above, or use the button below. You can also start a return request from your Purchases in My eBay.
Start a return - opens in new window or tab
"There are two ways to be fooled. One is to believe what isn't true; the other is to refuse to believe what is true.”
Søren Kierkegaard, Danish philosopher (1813 - 1855)