>>I am looking at the Madagascarophis colubrinus up for sale and was curious at the venom strength.
Sort of an uknown last I checked. There is one bite report in a field guide that I have not read. I gather it sounded worse that what you typically hear from Mangrove bites but there are always individuals who can have bad reactions. A sample size of one is not that useful here.
Also, it's highly unlikely that the M. colubrinus for sale are actually M. colubrinus. I have seen a lot of Madagascarophis for sale and there were only two that were M. colubrinus. The rest were all M. ocellatus or possibly M. meridionalis.
Regardless, they are cool snakes. But I encourage you to key yours out if you get them. It's very easy.
I have posted the key here on Kingsnake. A search should turn it up.