You may want to think twice before writing that $25 check to your favorite charity.

Small gifts often barely cover the cost of soliciting that donation. As a result, many nonprofits will turn around and sell your information to make a profit, according to watchdog Charity Navigator.

Some sell or "rent" their donor lists directly to other charities, but others work with middlemen who then sell the donors lists to countless other organizations. The charities buying your information will then likely pepper you with their own solicitations.

"You get put on a sucker's list," said Ted Claypoole, a privacy attorney and co-author of "Privacy in the Age of Big Data."

To avoid ending up on the list, you can either write a much bigger check (charities tend to closely guard the personal information of their largest donors) or check a nonprofit's privacy policy before donating.

According to Charity Navigator, nearly half of nonprofits require donors to opt out of sharing their information or they have no policy at all.