Just tell them you need to get rid of a dead body.I can see how that'll go - hello, I'd like to buy the 2 tonnes of sodium hydroxide you picked up last week, how much could I get it for ? - what are you going to do with that much ? Me - make soap, lot's and lot's and lot's of soap - you must bathe a lot.if that's too much - how much for 1 kg ? I can collect it.
Or that you want to cut it with some meth.
Then say, naaaaaah, just kidding, I'm a soap maker.