Why can't I bid any amount I like?

Bids move in steps, and the step gets bigger as the price does — so a cheap item is not bid up one CRETA at a time and an expensive one is not held up by trivial raises. Roughly:

• up to 1,000 CRETA — steps of 10
• up to 10,000 — steps of 100
• up to 100,000 — steps of 1,000
• and so on, each band ten times the last

The plus and minus buttons in the bid window move by the right step for the current price, and you cannot go below the minimum the auction already stands at.