Your boa has probably had problems since before you received it. If you're seeing the backbone like you stated, and it gurged on you. It's PROBABLY wild caught regardless of what the seller told you.
First thing you need to do is get prepared for the possibility of losing your first snake. Second thing you need to do is contact the seller and ask them what they are willing to do if the animal dies. Actually, do this first, contact your local herp vet and let them know you'll be in tomorrow to have your snake's health checked and that you'd like some dewormers and instructions on how to administer them.
Then, follow the instructions your vet gives you. DO NOT FEED YOUR BOA. I don't care how skinny it is. Boas do not die of lack of food. Next, adjust your boas cage temp so that there are no hot spots above 86 and nothing cooler than about 80 for right now. Too much heat or too much cool can cause more problems. Finally, follow the advice previously given about feeding. SMALL prey items for the next couple of months. After the second or third meal AND after you've finished the rounds of dewormers, get your boas feces checked. Everything should come back nice and clean (yes, I did just use the words "clean" and "feces" in the same sentence).
After everything comes back clean, start stepping up the size of the prey. DO THIS GRADUALLY. DO NOT OVERFEED YOUR BOA (regardless of whether it has had problems in the past or not). Every 10-14 days is plenty.
If you have any other problems. Come back here and state what you've done, and what has happened... and we'll give you a hand.
Hope this helps and I KNOW it comes off as derogatory or condescending, but I'm RUNNING out the door right now and I don't have the time to paint a flowery picture! lol, sorry
jb
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Jonathan Brady
"Sarcasm is angers ugly cousin" -Dr. Buddy Rydell (Jack Nicholson) in "Anger Management".