I'm not sure if it helps but in my experience I get more non-paying buyers on auctions than I do on buy it now listings. I now only have buy it now listings and I haven't set up immediate payment required because the majority pay withing a few days 🙂

I'm not sure where your non-paying buyers came from or if there's a pattern on the types of items they're purchasing but it might be a good idea for you to set up your listings as Buy It Now with Immediate Payment required. 25% is a really high figure.