The pinball gods move in strangeways. (not Nino that is!)...
Simple - Always have some cash aside to buy machines, and never use it to buy anything else ! Keep saving, and when an opportunity arises, don't hesitate. Half the fun of the hobby is the thrill of the chase, whether it be eBay or privately. You miss some, but there are always other opportunities. Be prepared to move a machine on if you have to. Use eBay and sell stuff you don't need!