Start from what it cost you
Add up what one item really costs you: buying it, transport, packing, and a little for the rent and your time. That's your floor. Selling below it means paying the customer to take your goods.
Add what you want to keep
On top of the cost, decide the least you're happy to keep on each sale. That number, cost plus what you keep, is your lowest price.
Your normal price sits above it, so you have room to bargain.
Come down slowly
When someone asks for a better price, don't jump to your lowest. Come down a little, and ask for something back: pay now, or take two.
Only call a price “last” when it really is.
Write it down
Lowest prices kept in your head get forgotten on a busy day, and staff can't see them. Write them down.
With Guavar, you tell your salesman the lowest price once. It bargains for you, slowly, and never goes below it. Customers never see it.