When is it justified to ask eBay to cancel purchase?

[AteRiri]

I purchased a jacket a week ago, and the seller has not sent the item. I have also sent the seller three messages, asking about tracking details.

Check your sales record against your purchase history. This should give you an estimated date for delivery, if this has passed you should be able to follow the options and tell eBay it hasn't arrived. Do this and you will open a not received request (aka case/ dispute) if the buyer can't prove it's been delivered or on the way after 7 days, eBay will close the case in your favour you will get a full refund.