To me this would not be much different than selling tires at the shop.
Tire margins – just for the tire, the labor is separate – typically need to be 20% to 35% to be reasonably profitable. That means the markup would be 1.25 – 1.54. Some shops run lower but if you look closely you will usually see that they are just trading dollars around leaving no profit or leaving a loss when all is said and done.
In your case the cost of shipping, PayPal and Ebay would have to first be added to the tire, then apply the same formula.
To be fair, the online margins probably should be higher since you don’t have the possible related service sales that you do at the shop. I’d be cautious about “competing with the other guys”. Some are very heavy hitters and others are on their way to going broke.