Will I be successful as an eBay seller if I started an online shop with them?
No, you will not.
Selling anything online at a profit is very difficult. High fees, low margins and you bring absolutely nothing to the table. I sold a few things here and there way back when but I did not make much and when I made a little, it did not last before I lost my source or others undercut me.
You're right. Your mental health isn't so good. Why would anyone buy from you when they could get the same things from a local store, and probably cheaper too.
Consider what you are doing. That is a business. So you need a business license which you have to pay the city you are in...go to city hall-licenses and they will tell you what constitutes a business and ask WHERE YOU LIVE because where you live may be designated "Residential" meaning "No businesses" The city does have ByLaw enforcement officers that will SHUT YOU DOWN.
What makes you think that anyone would want to go through the time and hassle of buying and paying the shipping charges for the parts that you mentioned that you want to sell on eBay (spark plugs, tires, cables, etc.)? He can go down to his local auto parts store and pick them up in less than one hour. at a cheaper price than he would have to pay you; or he can go to a local wrecking yard and buy the larger parts.
It depends on your skill level. If you are going to buy up cheap part and refurbish them and can do quality work, you may be able to make a go of it. But I would ease into it, where you only do it as a part-time venture right now and if you become successful devote more time to it. If you are going to just try and resell parts, your chances are significantly less unless you can get quality After Market parts at a significant discount. Otherwise you will just be trying to beat out the dozens of other sellers who are doing the same thing.