If you have good people judgement, you will be able to find the right home for them. I rehomed two pigs on Craigslist and both went to good homes. One lady drove nearly 2hrs to meet one of them! She ended up taking her home that day because I felt she would be a good home. The other was pretty close but we talked about exactly how to care for the pig before she left me. She came with a cute littele carrier filled with hay for her to hide in for the ride and sent pics of her cage and such. I had to talk to several well, idiots, before I found these people though. But good people are out there looking for a good pet. Be specific in your ad on how you want them housed, etc so it thins out all the irresponsible owners. Ask for pictures of their setup, and all that. It won't be as easy as just handing them over to anyone who asks, but obviously it's worth it.
Forgot to add: People who contact you with lots of information about who they are, why they want a pet, etc are the ones who are more likely to be a good fit. Those who simply reply, Hey i want your guinea pigs. Not so much... When I was looking for a pig, I always talked about my current cage, pigs, etc and gave the owner more insight on why i would be a good fit. If you end up feeling like noone is worthy of the piggies repost the ad at another time.