my 2 cents
It takes a little more than simply "keystoning" or doubling your money to make a profit.
To begin, ebay and paypal fees account for + or - 20% off the top.
You will need supplies and the cheapest I ever got padded envelopes, labels, etc averaged out from quantity, case lots, to .50 ea.
Postage I found to be around $.85 - $2.25 ea on average
Now with the ebay discount structure you want to build up to, requirements include fast shipping and a serious push for free shipping, all of which eats away profit.
You need seller discounts, plain and simple. Once you work up to getting the discount, you have to earn good feedback and ratings to keep them. You no longer need just negative feedback, simply a few 1 or 2 stars ratings from your buyers will eliminate your top seller discount. Again, have just a few of your buyers think you should ship free, and they hit you with 1 star, youcould be done.
So you are either going to offer free shipping and make up for the costs with higher quantity sales or offer cards that buyers don't mind paying a shipping fee for.
There is also a huge push from ebay to offer returns. Death for the card dealer with the number of people who will buy your centered MINT card only to return a NRMT off-centered card for full refund.
I know this sounds rather bleak, I am just being realistic, sadly from experience of over 12 years on the Bay. ebay is an unloading and buyers market. A place in my opinion to dump what you don't want for short money and buy what you need at a discount. I do see hot hot items being bid up, but that is generally rare, not the norm.
Hope this helps and best of luck what ever you decide.
Steve
thanks guys for your responses. im not sure what i am missing(i know, i know im not the sharpest tool in the shed!) but if you buy an item for $1.00 and sell it for $2.00 to me you doubled your money. if you buy an item for $20.00 and sell it for $40.00 you doubles your money. If all my fees are covered by the shipping and handling charges and im NOT factoring into this, the cost of supplies, envelopes, etc..and you "double" your money consistently, how can you NOT make alot of money quickly? just wondering what im missing or if my thinking is off on this

again, im not the sharpest person here on the bench, thats why im asking you guys for your opinions/thoughts.thanks guys