Ok so is there any way Britain could be persuaded to sell her West Indian colonies to the US before emancipation?
It's hard, unless you mean before the
British emancipation. I mean, the South would stop any purchase of free soil lands mostly inhabited by black people, many of which are former slaves or sons and grandson of slaves. And Britain may not sell without a clause explicitly barring re-enslavement.
OTOH, the islands were still too valuable for Britain to sell in the first half of 19c., especially before abolition (that happened in the late thirties, IIRC). It is debatable when the US are going to be able to pay the considerable sum required.
Britain might chose to get rid of the embarassing slave-holding colonies selling them to another slave-holding nation, though I doubt the British public would be happy at that. But THAT would mean a whole bunch of new slave states and a GREAT amount of black population, both slave and free, joining the US, something the North is going to oppose vehemently.
So, while not impossible, the reasons behind such a move are to be explained and the scenario may require an earlier POD that eases the matter on both sides.