Just keep a cool head and don't let the excitement of a big sale cloud your judgement.

If they are unable to give you the cleared funds today, then don't let them take the item today. Just don't. If they have to wait until Monday then so be it.

And, if they do end up giving you cash, make sure you count it out slowly and carefully in front of everyone. Don't let anyone rush you.

And make sure you have someone else with you when they come so that you have another person on your side in case the buyer tries to push you to do something you don't want to do.