NP fuddj. I think that I need to do some clarifying myself ... this:
Is not some complex algebra; it was supposed to be an italicised x, x :)
Also, to expand a little on what I was saying about calculating potential losses, it's not that I feel one should plan on selling at 30-50% of the average eBay price -- basic market dynamics won't let that continue for long. What I am saying is that in the real business world, most companies accept that there is such a thing as cost of acquisition when it comes to building a customer base -- ie. that every new customer is going to cost them a certain amount of money to actually get on board (advertising costs, infrastructure, incentives, payroll, etc.) and the goal becomes not how much money to make off of each customer in the door, but how to best keep them coming back.
When starting out on eBay, over and above price it's incredibly difficult for new sellers to build trust. Many buyers will buy from a seller with 2,000 feedback at a higher price than they will buy from one with 0. But if you have done business with that particular customer before, the relative strengths of your feedback score to the old guard powersellers becomes irrelevant.
Just looking at the gross profit margin on one sale of a given product is not the best way to gauge how, where or when to enter the market for that product. See what you can profitably upsell, how long you could function at a marginal loss, and make sure that you have everything you need to deliver 100% stellar customer service to your new customers. Providing top notch service adds a lot to your costs too, not so much in dollars but in time, but it's an additional cost that needs to be figured in -- and like negative margins, it's a cost of which you shouldn't be afraid, just realistically calculate how much you can stand before you start getting hives.