Most petstore boas aren't pure colombian. I have seen many boas sold as colombian that were clearly full of central American blood. Now on to the size. Assuming it isn't B.c.c., you likely won't end up with a 10' or even 8' snake unless you try for it by feeding a lot of food. Its rare, even with power feeding for a B.c.i. to get much over 10' and in my opinion shouldn't even get over 8'. There are a ton of localities of B.c.i. with some that will never get over 5' even with a lot of food to the ones that can occasionally get to the 10' mark.

For $350 with cage and all, I would pass. I would much rather start with a guarenteed(sp?) healthy animal that you can raise on your own. I would have been fairly intimidated had I jumped into the game with a 6' boa vs. a 18" baby that grew into a 6' boa over the period of 3-4 years. You'll be far more satisified if you start with a baby vs. an older one. Starting with a baby, you should have no fear of a giant snake since you have control over this when they are babies.

Hope this long post helps