[You're quite right there is no evidence for what I say but there isn't any for what you say either.
Well here's the evidence for the College's finances being sound: http://www.camsfc.ac.uk/assets/file/Annual_reports/Annual_Rep_08_09_lr.pdf
In addition, a fairly recent Ofsted inspection graded their financial position as 'outstanding', I believe. So we know that the College is not disposing of the site because they are in any financial difficulty.
In answer to your question....
HOW DO WE KNOW THAT WE WON'T END UP IN TWO YEARS WITH A VANDALISED ABANDONED SITE?
... we don't KNOW anything for sure, but that outcome is so improbable that we can safely disregard it. This is because the College can not afford to (indeed, would not be allowed to) close courses or deliberately reduce student numbers. So they will not sell the site unless they can get enough money for it to build sufficient replacement accommodation on the Buxton Lane site. And no-one would buy it for £12 million or any other significant sum and then let it become a wasteland. I understand the point about the site being bought up to be a 'land bank', but AFAIK that is usually done as a 'blocking move', when a big supermarket chain (e.g. Asda) buys up land to stop another big chain (e.g. Tesco) from opening up in competition with them. The only supermarket in that position round here would be the Co-op. Can anyone see them doing that? Me neither......
Where I do agree with you, finetime, is here:
We can't know for sure until we see agreement details but what has probably occurred is that Asda have bought it subject to obtaining PP and if they don't get it ownership reverts back to MCSFC.