Hah...yes, that is definitely a "depends" answer. Here's the factors you should be asking yourself:
- If you are using credit cards as a way to fund, figure out a level you are 'comfortable' with. Your goal should be to make sure you can sell everything off BEFORE the credit card payment is due. If all you feel comfortable with is $200, buy $200 worth of stuff and get started.
- As far as the number of units to buy, its all a matter of the strategy. You could go the 'thrift' strategy, and go to thrift stores, garage sales, and closeout areas of big box stores to buy whatever you can make money on. Since this strategy really isn't scalable, it should be a 'startup strategy' to get your feet wet, and to get some working capital. Once you have working capital, buy the MOQ (minimum order quantity) from whatever supplier you choose for products. In some cases, it may be cases of 6, 12, 24, 48, 72, etc.
- When you are starting an ecommerce store, the number of items and niche factor are a big deal for branding purposes. They can make or break a customer from buying from you. On amazon and ebay, selection and niche doesn't matter for branding purposes. Amazon and eBay are the brand - not you. So in reality, you can pick and choose whatever you feel like selling. Just start off with SOMETHING, even if it's just one SKU.