You will need to consider costs very carefully. It is easier to work backwards i.e. What is the bottom line you need to earn
You will need to get public liabilty insurance which would need to cover liability to customer if product fails i.e. if someone buys tyres off me and one blows up killing someone, I am liable at first as customer purchased from me.
Do you have time to run 2 jobs?

most companies will require initial stock order to get trade discounts etc.
What sort of thing do you want to sell?
Who will buy it?
Will the market get bigger or shrink?
What to do about posting goods out?
What about returns?
What about TAX and becoming self-employed?
Stock holding premises or warehousing?
These a few extra items to consider