This is a difficult question to answer, as it is not simply one or the other.
I have had multiple people message me on ebay about “tips” on how to sell pokemon cards for a living, needless to say, there isn’t a secret. It is the same boring old answer for all businesses: having more knowledge, experience, capital and patience will help, but there is no guarantee.
I would echo what some of the other AU members have mentioned, consider your region and what the monetary costs and overall opportunity costs are in relation to buying and selling. Also, if you are full time as a “business”, typically that means you are going to be paying taxes. That is what separates the men from the boys (actually full time business vs occasional sales). Nothing against guys who don’t pay taxes, as most people who don’t are selling occasionally or part time. However, if you want this to be a full time venture, taxes are a significant cost you will have to quantify. I am not sure what the tax laws are in AU, but am stating this as a universal response.
There really isn’t an answer on “how many items you sell” or even “how much you sell”. For example, you could have a year of say 100k in sales and live off of that as an individual. If you are a high volume low profit seller, you may need double that to make a living wage. It depends entirely on your costs and profit margin. If you are doing primarily new products, the profit margin will be smaller, and you will have to increase volume.
Either way, there is no gravy train. Even when people see a high end item sold, sure, the profit is fantastic, but the amount of time, work, effort, and perhaps luck that went into that sale is significant. Like everything else in life, there is no real shortcut. If you are taking one, there is a shadow of risk or illegitimately looming (shill bidding, lying, avoiding taxes or other legal expenses, etc.).
Basically this is like any other business, except you most likely enjoy the product. The business aspect is no different than any other retail operation. The general answer is the more knowledge, experience and capital (money to spend for your inventory) you have, the better off you will be.