Big question!
What would be helpful as a starter is to get some idea where your ambitions lie for your “side line”. Are you happy making a nice few thousand here and there or would you want to grow the business and maybe make it the next Amazon?!
Is it a relatively risk free business or do you have exposures from the goods you sell?
Any chance of making losses or is it predictable profit of one size or another?
Would your total earnings push you into higher rates of tax if you just go the self employed (not company) route?
Do you want to share the business with family or friends?