Let’s consider a couple of variants. If you are an independent manufacturer, the choice of products is not a problem as you sell what you produce. Nevertheless, as a rule, you are not (well, actually you may sell some DIY stuff, but this is not the case of real trading via eBay, put selling single items and we are not interested in such sales). So you major problem is to find a reliable source of products. It must be truly reliable, I mean that the goods you are going to sell must always be available to you and thus to you customer. If you are not a reliable seller, the clients are likely to find it out sooner or later and you will be a roaring failure in your business.
What else do you need?
Well, I would say that there are two ways from this time on: to stock products somewhere or make the manufacturer stock them and use only as required. How can this factor affect your business? It is obvious – your stocking the product will inevitably affect its price (you have to pay rent to the warehouse you use). Of course, you can stock them at your own place, but as your business grows bigger, just imagine how jammed it may become. And if you pay the rent you have to make the price of the items you sell higher. It might not affect the goods that are sold by great numbers (some small stuff), but if we are talking about custom items? Each item will cost considerably higher, and you are likely to become uncompetitive in the niche.
Another way is to find the reliable sources of goods that stock products themselves and you take what you need as your customers make orders. In this case the working scheme is something like that: you find a source – place the products on your eBay store – your clients purchase something and pay for it – you receive the money – you order the products from your source – the source either delivers the products to the customer directly or you have to do it. The difference between what you get from your client and what you pay to the source is your profit!