Selling house - what are the terms now?

Hi all, we're selling our house and have received an offer just below market value. House is in Logan QLD. Our concern is the longer than usual settlement terms, the buyer needs to sell their acreage property so needs 40 days subject to sale to go unconditional, and then another 30 days, bringing settlement to total of 70 days. I asked my solicitor and he has said it's longer than usual and puts us at risk because our property will be off market for that long and contract can fall through. Can I get thoughts on this? Thanks