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Here are a few questions I would ask you regarding starting a knife-selling business-
1. Do you have any experience running an online business, or any business, or much experience simply selling things online?
2. What type of knives would you sell? There is a wide range of knives out there, from cheap Home Shopping Channel stuff, to high-end knives that cost hundreds of dollars a piece, or more.
The type of knives you sell would determine how much competition there is out there. And in order to be competitive, you would have to price your knives at the same price or lower than the competition. As a result, your profit margin could end up being so small that it's not worth the trouble.
3. What type of payment system would you use? That gets complicated. Credit cards? PayPal? You're talking about legal/tax/IRS/government stuff there, including a business license, and possibly some type of liability insurance (like in case a kid buys a knife from you and cuts themselves and their parents want to sue).
There's a big difference between running a successful knife business, and losing money trying to turn ones interest in knives into a business. To run a successful business you first need to have all the legal stuff down tight. Otherwise you could end up with crippling tax problems, fines, and possibly lawsuits, and at the very least, maybe a lot of inventory that you can't sell.
Then you need to have your "numbers" down tight. You need to know EXACTLY what it's costing you to run the business, every last thing from the legal stuff (license, insurance) to the cost of merchandise, to the ongoing cost of running a website, the cost of dealing with returns and refunds, etc, etc, etc.
I'm sure I'm forgetting some stuff (or a lot of stuff), but you get the idea. Running a business is a complicated matter. You have to cover ALL your bases, and ALL the details, because if you get one thing wrong, you could end up losing a lot of money.