This is an easy one. Pick up a nice modern stern pro that comes along that is a good deal. Do the same with a mid 90s Wms. If you look for a certain title they never come along or they are always overpriced. If you wait for the deals to come along you can play them and flip them with little or no loss.
My next suggestion is buy the game you really want. For example, if you want BM66, buy it then save some more till you can get another game.