Hi Tanya, I should start by saying, welcome back :)
In regards to the profit margin you suggest that is based on spending $38.00. I'm not sure how you can up with that equation, but that's certainly an impressive profit margin if you can turn $38.00 into $350.00. It's not impossible of course, but it will all depend on the market itself as to what your profit margin is going to be, but good luck with it.
You're idea about using your profit to roll back into your business is smart business sense. Rolling profit back into your business is how you build a business, especially from a low starting base. The mistake many make is that they only ever roll the principle back into the business and draw the margin out straight away.
That of course means that you will only have the exact same amount to spend on the next lot as you did previously, so there is no option to build the business. If you can roll whatever margin you make back into the business by way of increasing your stock profile, then you start increasing your business base, which in turn then starts to increase your margins.
It's a compounding effect!
So while it's very tempting to keep pulling the margin out and rolling the principle back in, that's akin to only paying the required minimal monthly repayment on the credit card and only ever paying off the interest, but never making a dent in what you owed to even start with ..... the credit card companies LOVE that btw.
$250.00 a week is possible, but not easy in a super competitive market. I think you're going to have to really get smart with your advertising copy, build up your brand image to give consumers as much confidence as possible to deal with and really focus on your marketing.
Most people making that sort of cash flow per week have a lot of boxes ticked and they spend a lot of time staying on top of that as well.
In regards to companies that charge absurd amounts of cash for so called secrets of online sales success. Well, I've made my own personal views on these types of schemes well known here on the forum plenty of times and nothing has changed. There are no secrets to being successful in e-commerce, it's just a good understanding of business principles and market forces, nothing more.
The biggest problem is, that many people would rather pay absurd amounts of money to be told that there are secrets, rather than be told the truth, because the truth is never as exciting and as enticing as these snake oil peddlers make it out to be!