start a price check thread here if you really have no idea. From your history you tend to shoot high and then not sell from what I can see. If you really want to sell you will need to adjust this habit.
For a general rule, spend a little time researching and you should be able to hit spot on. mrpinball classifieds, ebay completed listings, pinside completed listings, and boston pinball guide are ok benchmarks to then adjust from. You should be able to figure it out; afterall you did buy them all at once and I assume managed not to get ripped off
Most importnatly I woudl say you know what you paid for them, so if you got them a few years back and have enjoyed them then you are going to do just fine. Even if you undersell from FMV you will probably still be well over purchase prices form a few years back. Aside form that, selling for a fair to good price can feel really good. Nothing wrong with hooking up a good local pinhead and writing up an agreement for right of first refusal to buy back at same price if sold within X time. I know others that have done it.