Not sure how to answer that question since it varies by each situation. However, I was taught and learned through experience nothing is sold until it is bought. Or, I can be selling something for a moment or hours on end. Buying too can be a moment or days on end. The bottom line again is nothing is sold until it is bought. That action is barely a moment.
You are right. Nothing is sold until it is bought. But if you are buying to resell. What would be easy for you. Giving extra money for something you buy Or receiving less money for something you are selling?
Buying and selling online are getting easier and often cheaper than more conventional methods, and takes a lot of the hassle out. And the best part is you can compare to many such similar things with prices. Also if you want to sell something, you will have access to a world audience online.