There are very few reasons that I can see to cause your validation not to work is that this code is never called or it is called after the hook is actually run.
add_filter('acf/validate_value/name=sell_price', 'allow_only_floats', 20, 4);
add_filter('acf/validate_value/name=buy_price', 'allow_only_floats, 20, 4);
Another possibility is that the names you are using are not the names you need to use. This would be the case if these fields are sub fields in a group, repeater or flexible content field. Have you tried using the field key variation of these hooks?
