I'm not sure what to do next, so I'd love some advice.
I've been on ebay for over 10 years and never had a problem like this: I purchased a mirror (brand new) from a commerical seller. When I received it the whole mirror was cracked and thus unusable and not in the brand new condition the seller described.
I messaged them, asking to return the mirror. They told me I could do that and they'd send me the shipping label. But it's been 10 days and no answer anymore. I've sent two ebay messages, asking them nicely to respond but nothing =(
I cannot use this mirror and I'd really like to return it and get my money back.
What should my next actions be?

You can file a dispute for SNAD with ebay (I think they are best for SNAD), and perhaps then the seller will respond. Sorry you received this item broken :(.

I hope all your messages to the seller have been thru ebay's system..

You should now file a SNAD... and sorry that your seller hasn't been more

communicative..

I agree, file a SNAD with ebay. You have given the seller plenty of time and for whatever reason, they stopped responding. Opening a case should get the sellers attention and hopefully resolve this for you.

In addition to which this goes against the seller