Barcodes Questions

Hi all, a hobby of mine (customizing hi-tech products) is surprisingly doing well, and I consequently need to be way more compliantly than I am.

Pretty simple process: once I get an order, I purchase the product, I open the box, I customize it and send it to the customer.

My questions could be very ignorant, so pls bear with me:

• should I add a new barcode or use the existing one?

• if I decide to sell (other products of mine) on Amazon, are the barcodes different from the 'normal' ones?

• What's the best website/place to buy barcodes?

• I am sending all over the world: are barcodes different per country/continent/etc?

I told you the questions were gonna come from an ignorant stand! Thank you all!