I seriously doubt that either one of those machines will go for $500. Either one has a scrap value that exceeds that. I think that scrap iron goes for about $0.40 per pound. A Monarch 16x30 weighs 4800 pounds or more (depending on the model), so it's worth almost $2000 as scrap iron. A guess is that the 18x120 weighs close to 10,000 lbs. The shop owner might take less than scrap, just to keep a faithful old machine from winding up as rebar, but he might not...
If you haven't spent much time on eBay, be aware that there can be several people planning to bid on the machines that are using "sniping" services, such as esnipe. They allow you to set up a bid that will be automatically submitted seconds before the auction ends. But even if you won it for $500, the seller can cancel the bid and scrap the machine for more money. If you want one of the machines, figure out what the maximum amount that you're willing to spend and put that amount in close to the end of the auction.
Cal