I've been offered a lincoln electric 305c and it's not too far from me. It's up for £400 but the seller has no three phase for me to try. I'm really tempted but this really puts me off.
Get a receipt stating the item is working as it should that he signs for a start. Should it turn pear shaped this will be in your favour. If he refuses you know it's a dud. You could ask about a 7 day return but I wouldn't agree to it if it was me as buyers can be just as bad as sellers.