So if your uncle has a shop front and you are helping him to grow the business with an eBay presence, nothing much will change from his normal avenue of sales.

No doubt he'll already be collecting GST from his shop, and you will need to do so on eBay.

He will already have expenses from running the shop. PayPal and eBay fees will be an extension of those expenses.

Taxable income will probably be in the form of company tax (assuming his shop is setup as a company) which is currently 28.5%.

Your uncle may not be good with the internet, but if he's been running a shop for a period of time no doubt he would be fully aware of how sales, profit, expenses, and tax work.