I bought one off ebay which had had some sort of glue in it, was told it was a water based glue which would easily wash out, but when it came it had a thick layer of glue all over the internal sides and floor which needed scraping off. In the end I had to cut the top of the ibc off and get inside it to clean it.
I reckon this might be the only way that you will be able to clean yours properly.
Lesson learned though, never buy used ibc's that have had glue or oil based products in as they are impossible to clean properly without taking the top off.