I sell lego and always try to make sure I remove the mega blocks first. I normally sell them seperately but not as lego. they aren't worth as much, at the moment and although compatible with lego don't always connect correctly - even with other mega blocks.
Beware that some people - a few may not realise I suppose - put mega blocks in but don't tell you.
Always ask the seller first if in doubt! If they lie at least you have proof they lied!
Good luck - reliable lego seller.