Buying shares from an LLC, is this fishy?

[Rod Saleh]

I'm looking to buy shares of an LLC. The LLC is owned by a trust and a couple of investors. The trust is looking to sell 1 share to me. The owner of the trust says he'll personally guarantee my money back within 6 months if no cash flow from biz. He wants to give guarantee with a notarized memo. But he says he can't put that guarantee in the purchase agreement because the other investors wouldn't agree to it. Can he guarantee my money with the memo? does this sound fishy?