From the eBay help pages:
If your item arrives damaged, doesn’t match the listing description, or is the wrong item, you can return it for a refund. If you've changed your mind and want to send it back, you can ask the seller if they'll accept a return.
https://www.ebay.ca/help/buying/returns-refunds/returning-item?id=4041
If you no longer want an item, you'll be able to see in the listing if the seller offers returns, how long you have to request a return, and any other conditions. You can check the listing by selecting the item in your Purchase history.
Re-selling the item on eBay is also kosher, but newbie sellers are more vulnerable to less than ethical buyers than more experienced sellers. It may also not be worth listing a single item versus the return you'll be seeing when all your fees, time, and trouble are taken into consideration.
