Sure happy to help. Your email in your profile appears to be invalid, but I'll write a few things here in case other folks are interested.
They become big because the competitive edge is not in the supply, it's in the demand, as in, they have access to a customer base. Of course, one can't sell complete crap, supplies/products should fulfill a certain market demand, but supply is always secondary to having access to customers.
If you're thinking about becoming a trader or distributor, start backwards from your potential customers. Who are they, what do they want, and what price point and average buying quantity makes sense for them. If you're trading B2B that is quite different from sourcing from a supplier to sell direct to consumers in your market. In the former, your customer is the physical retail stores (or their equivalent), in the latter it is actual consumers themselves. The usual customer profile work should be done here.
Once you have an idea of your customer profile, then you can start sourcing. Luckily there are very few barriers to entry in 2021, you can google for product names or categories, find the suppliers online (China or otherwise), and you can just email them to inquire on their prices. If you're starting out, be a simple as you can:
a) What are their prices and minimum order qtys
b) Can they waive the minimum order qtys while you test out the market on the first few orders (suppliers always have excess stock on previous production runs. You can do them "a favour" by buying that - they recover their costs, you get things for cheap).
c) Pre-sell if you can. That could be as simple as walking to stores with a catalogue, or just a sample to show them. Then collect pre-orders from 50 customers (ideally with some money down), use that to make your first supplier purchase.
d) Ideally, you have exclusive rights to the distribution for several years. When you're starting out, you don't want to spend the money to "buy" these rights, so again, it becomes a negotiation. The supplier isn't selling in your market, so you're helping them reach a wider audience. It's in their interest to support you.
e) Make sure besides exclusive rights, you get collateral support in terms of marketing materials, graphics, ads, freebies, etc. This stuff is huge when you're building up a customer base.
The best structure is pre-sell, purchase from your supplier to fulfill existing orders, then the stuff arrives and you just pack it off straight to your customers. Rinse and repeat. High volume items give you about 10-15% gross margins, but standard is about 30-40% gross margins as a trader. Again, all depends on how you negotiate your total costs. Freight is often a huge win -- get some good freight charges, and your profit can literally double on a specific shipment.
Ideally in your customer identification process, you'll have found some product ideas that they'll want to buy. You are valuable because:
1. Your customers don't want to buy the volumes a supplier would require, they just want a small subset of that total. They're happy to pay a larger price for smaller quantities.
2. Suppliers don't want to sell bits and pieces, they like having someone buy large quantities from them. So they'll give you a good price for bulk purchasing. They also don't want to do the legwork to find the 100-1000 smaller customers to fill a large production order.
Your profit is the difference between the two numbers. So you do the legwork to get that upside.
If you're attempting to sell on crowded locations like Amazon, then you will need to spend/invest on ads to drive the purchasing from you (vs. someone else with the same or similar products). Most marketplaces reward sellers with existing sales volumes, as their algorithms figure you've completed sales before, you'll continue to do so. So that's a game you do need to play to start building a seller reputation. If you have your own Shopify-or-whatever site, it's the same, you'll need to do FB/Google ads to drive that initial customer acquisition.
There's a lot more info, but I'll stop here for now. Happy to write more if folks are keen.