Hi Axe,
Everything you incur in operating your business is tax deductible. That would include the merchandise you buy, ebay fees, paypal fees, vehicle expenses, internet service, computer repairs/payments, office equipment/supplies you buy, buyer refunds, postal/shipping fees, packaging materials, phone, office space, etc., etc.. Of course some things are fully deductible (like listing fees, merchandise purchases), while others are partial (like phone, utilities). For vehicle expenses, you need to get yourself some kind of mileage log and each time you drive somewhere for your business, make note of the mileage you use, use your car odometer and keep track of the total mileage you use in a given month then divide your work portion into the total amount of mileage and you get your percentage of your total vehicle expenses that is tax deductible. Or you can use the flat 40 cents a km deduction as well. Either method is legal. Vehicle expenses would include, lease payment, gas, repairs, license and registration.
Another big thing is gst/pst. If your an official business you need to be collectiong gst/pst on your sales within Canada and within your own province as well as the Maritime HST. Personally we include it in the price. But the good side of that is you can claim back all the gst/pst you pay out in operating your business against what you owe and ebayers I would think in most cases, since they ship mostly outside of the country as well as outside of their own province, get a refund every quarter. We get a nice big cheque, which of course is always nice:-)
If you have an accountant friend, he or she can help ya out, but for now that'll give you an idea.
Best of luck!