There is no secret supplier. Stores that sell steadily buy from ordinary places and are disciplined about price.
The six that actually work
Your own things first. Everybody has a cupboard. It costs nothing, it teaches you the whole process, and you find out what sells before you risk money. Start here even if you intend to be a reseller.
House clearances and estate sales. Mixed, unglamorous, and where a lot of good stock comes from. You buy a box and sort it afterwards. I inherited a collection — where do I start?
Local markets and junk shops. Slow, physical, and the best education there is. You learn what things cost by handling them.
Wholesale, for new goods. Minimum orders are the trap: a hundred units of something you have not tested is not a bargain.
Other people's auctions, including here. A bundle bought and split is a normal trade. Can I buy things to resell?
Makers, if you are reselling handmade. Agree terms in writing, pay promptly, and never represent their work as your own.
The rule that matters more than the source
Buy at a price that survives a disappointing sale.
An auction finds the price two people will pay, and some nights that is less than you hoped. If the only version where you profit is a good night, you will lose money on the ordinary ones.
Work backwards: what it realistically fetches, minus the commission on the item, minus packaging, minus your time. What is left is your buying price. Why did my item sell for so little?
What to buy at the start
Things you understand. You will be asked about them live, on camera, by someone who may know more than you. Nothing exposes a gap faster.
Small and light, while you learn delivery. A bulky first mistake is expensive twice over. Why does a big light box cost so much to send?
Varied, in small amounts. Ten different things teach you more than ten of the same.
What not to do
Do not buy in bulk before you have sold anything. The commonest expensive mistake.
Do not buy what is selling well for somebody else without asking why it sells for them. Often the answer is their audience, not the item.
Do not buy anything you cannot legally sell — and check before, not after. Prohibited items explained
Letting demand tell you
Enquiries are free market research. Three people asking for the same thing in a month is a buying decision that arrived without you paying for it. Can I ask a seller to find me something?
More: can I buy things to resell, how to price for auction, what does a realistic first month look like.