There's no set percentage for feeding boas. We usually say to feed 10-15% of body weight for BPs but to choose a prey item that leaves a slight bulge in the belly. With boas, you do NOT want a bulge in the belly (an occasional very slight lump is fine). Smaller is better.

I tend to agree that buying from a reputable breeder is the best course of action, however, you can make some real finds on Craigslist (although I've never been lucky enough to come across one....). It seems like you have some experience with BPs so you'll know what to look for in a healthy snake when you go check out that boa. Here's some extra tips to watch out for though if you do go...

1) Make sure the snake is well-coordinated. If it wobbles or tends to "stargaze" or doesn't grip you strongly or has the least bit of trouble righting itself if turned onto its back, WALK AWAY (or ever RUN away...). These could be signs of a neurological problem or even IBD.

2) Boas should be more slender than BPs and have a leaner and more square appearance. They will often look skinny to those who have mostly BP experience. That is ok. But it should not look scrawny or weak or sickly in any way.

3) Use common sense. Other than appearing thinner than a BP would, a healthy snake looks like a healthy snake no matter what the species. Stay away if anything gives you pause.

Good luck and let us know what you decide - we'll want to see pics!!