What workflows or services can help me figure out the right price for my items and negotiate effectively to maximize my profit when selling online?

We ask the same underlying question in different ways.

• What workflows or services can help me figure out the right price for my items and negotiate effectively to maximize my profit when selling online?
• I want to sell things online for the most money possible. Are there any services or strategies that assist with pricing my items correctly and handling negotiations?
• How can I make sure I'm pricing my online listings correctly and negotiating well enough to get the highest payout?