I'm fairly new to guitar pedals. I tried to buy on ebay used guitar pedals, but none of them are selling with power supply. Why is that so?
Mainly for two reasons - they mostly can work with batteries, they are originally not sold with power supplies. A third reason may be that the original owner will still have other pedals, so will retain the psu for his own use. If sold to another country, the psu may not have the suitable plug or even input voltage.
To expand on the point Tim made about the fact that they are originally not sold with power supplies:
When using simple 'stompbox'-type pedals, guitarists typically use a number of pedals chained together.
Rather than have a separate mains power supply for each pedal, it's more convenient to have a single high-quality power supply powering all the pedals - e.g. the Pedal Power 2 plus in this picture:
If you're powering pedals in this way, you wouldn't want the prices of the pedals you are buying to be inflated by the inclusion of another power supply you wouldn't need.