I think you are beeing too scared with your rummage sale idea. We used to have rummage sales to raise funds for the skating club I was in (as did most other clubs in town). Items were priced individually, even clothing. I also did a rummage sale in Urbana when I was moving to get rid of a few things. I sold my book cases for around $10, individual books for $1. Later in the day I was more open to haggling. Try selling clothing for $2/piece and other items priced between $5 and $10. People will haggle if they think it's too much, but will haggle less if they know it's for charity. If you still have lots left the second day, lower prices further.

As for cookie sales, I've tried that too now that I'm raising funds for the AidsRide. I've been bringing in little bags of home made cookies to work and selling them for $2/piece. I've made $200 so far and people keep asking me to bring more in. I have also gotten a few extra donations this way. Probably another $200 so far. It's a good way to advertise what you are doing.

Good luck!