I did a Google search but didn't see that this had been addressed. Is the chance to sell a magic item per month cumulative? The text seems to imply this:

EXAMPLE: Using a formula, Quintus and his assistants made 4 scrolls of fireball (base cost 1,500gp) at a cost of 750gp each. Quintus has a henchman take these to sell them in Arganos (a Class III market). There is a 25% chance of a buyer for one scroll each month, at a price of 3,000gp each. Quintus also asks his henchman to sell a sword +2 (base cost 15,000gp) he found in an evil crypt. The sword +2 will sell for 15,000gp.

There is a 25% chance of a buyer for the sword each month. After four months, Quintus’ henchman returns with 27,00gp – 12,000gp for the scrolls and 15,000gp for the sword.

Bolded emphasis mine (BTW, is that an error in the text? According to the chart an item worth more than 10k should only have a *2%* chance of selling per month, not a 25% chance as the text states the sword has).