$5????????? WOW - that is amazing!!!!!!!!!!!!!
What a wonderful thing you're doing for these kitties!
What one of our members does when she finds a kitty with a collar (OK, so this is a ratty flea collar) is she writes a short note with her number on it and attaches it to the collar. In fact, for kitties that look cared for, she keeps collars around to attach notes to. Still has them spayed and neutered - but she works with a rescue, and if they can't find an owner, they help rehome the strays (but just release the ferals). Just a thought for the future.
Since you won't be ear-tipping all of them, I'd really recommend a notebook and a polaroid camera (or take the time to print out pics from your phone or digital camera). Believe me, when you and the cat have completely stressed out only to find it's one you've already had sterilized....
...as to being sued? It's only a problem if you're trapping on THEIR property. If kitty was left to roam, it is not an issue.
for the wonderful work you're doing!