Our 501c3 collects free food from local stores; we donate to local needy; we usually collect a donation; each recipient signs each time saying donated

The estimated fair value of donated items constitute contributions, and are generally in the nature of "public" support that substantiates your status as a public charity. The donations you make are program expenses. Since the "donations" you receive are not in exchange for tangible goods or services, but rather voluntary, sales tax will generally not be due on items you give away.