Your first step is to contact the seller, politely explain your problem and ask how he can fix it.
The fairest offer, to you and to him, would be that you return the item at your own expense (with Confirmation of Delivery*) and he refunds you the selling price but not the shipping price, which went to the post office not to you.
He may have another suggestion.
A second possibility, because shipping is expensive, is that you leave the eBay transaction alone and re-sell the item yourself, possibly through eBay but more efficiently through an online free classified site like CL or KJJ.
If neither of these work for you, you can open an Item Not As Described dispute with Paypal. Keep in mind that 'not liking the material' is not a legitimate reason for return, PP may still accept your dispute and when you prove* that the seller has the item back, Paypal will refund your original payment,but not the return shipping.
*If your seller is within Canada, any service you choose for a bulky item like a tent will be Delivery Confirmed/tracked. If your seller is overseas, tracked service starts around $40 (a 2.5 kilo parcel costs over $75 to ship)