Okay, so the ad you bought from will have had a delivery date range. 

Go into your purchase history and have a look at those dates now.

Write them down, take a screenshot, whatever.

You need to know exactly when your order should arrive. Once the last date has passed, you can go into ebay and open a proper claim for a full refund on the basis you did not receive your goods.

You can only do this though within 30 days of the expected arrival of the item.

That's why I said to take careful note of what it is.

 If you don't do it within that time, you won't get a refund at all.

You ask if you can cancel the sale. No, you can't.

It is still possible you may receive the item, the seller may surprise you, but the fact they are ignoring your messages is unsatisfactory in itself.

You ask if you have to wait the 6 weeks (or whatever the delivery date is).

Technically, maybe not. Depends on how you paid.

But I would definitely advise you to wait and to swing into an ebay claim the minute your parcel is overdue.

Not now, asap after the last delivery date as in the ad.

You should have no problem if the seller cannot provide tracking etc

Your other option, if you paid via paypal, is to opena  claim with them but again, I would not advise this yet. Ebay claim first and if for any reason that didn't work out, then try paypal.  You cannot do it in reverse order, you cannot do them at the same time.

I have heard of people opening a paypal claim within minutes of a regretted purchase and getting an auto refund on the spot, but that certainly isn't a given and if you try that, you would close off the option to open an ebay claim later if it does not arrive.

Wait out the time. As you have noted, you didn't check the seller out before purchasing, so you need to follow the eBay guidelines for a refund.

You might even get your item before your (arbitrary?) 6 weeks have passed.

No, only the seller can cancel the order - even within the 1 hour period after you pay.