Here's an example - for you that don't seem to understand:

You list a car for sale on eBay.
You want atleast £500 for the car - So you set a reserve of £500.
The listing ends at below £500, the car goes unsold.
Afterwards, someone approaches you offering £500 for the car.
You sell it.

See? It's not immature. Nor is it unprofessional. It's business.