Do the math before you list any item.
Here is an excel spread sheet that I used. Note the eBay fees have changed (i.e. no longer there is a Paypal fee and eBay fees have changed.). If you don't know excel find a friend that does. Heck I learned it when I was over 50 y.o.
"I have the right to remain silent but I didn't have the ability." Ron White, Fritch, Texas
"Stay away from negative people, they have a problem for every solution." A. Einstein
"The Devil made me do it!" - Flip Wilson
"If the band can only play loud - they ain't no good - peps too!" J.R. Johnson
