I'll usually hold a machine for $100 for others. Half seems a lot to hold a machine: I'd ask why so much and counter a smaller amount. But only after talking on the phone and getting a feel for the seller. But if they aren't a pinsider you can research, I wouldn't move forward with a large deposit.
I've done half once before for a machine: but that was a retailer I saw in person who was going to get it broken down, cleaned, and wrapped for me to pick up.