Hi Laura and welcome to the forum,
That's actually quite an interesting question, at what point does an income become taxable. Selling a few things on eBay is fine, I doubt the tax department would come knocking looking for their slice of that. However, and this is the technical answer, any income derived from any avenue is usually taxable.
So the technical answer is that everything you make should be declared for taxation purposes, but if you did that, you may well be the only person on the planet to do so. So there really isn't any line in the sand as to when you do start keeping books, the simple answer is when you register a business really.
Can you buy in bulk without being a registered business, well that depends where you purchase your products from. Where a surplus supplier usually won't require any proof of trading, a wholesale supplier usually will. So if you wanted to buy wholesale, then that would be the time to set up your business.
I hope that helps at least a little lol