eBay cannot help you here, as they cannot enforce the seller to reply to your message.
When did you contact them, and when did you buy the item ?
Unless this was a vehicle, I'm presuming you have paid? It's a bit of a grey area for a timeline, but once around 7 days have passed with no communication from the seller , you need to open a case for item not received, and if necessary, escalate on day 4 to receive a full refund.
It may just be a case of waiting a little longer for a reply, some peeps are just not the best at communicating asap to messages!
@laurcrossle_0
