Stu/Kevin,
Thanks for the honesty & insider's view of what it actually takes to get something done from concept to shipping. You guys do top-notch work! I haven't pulled the trigger on anything yet directly from you although I have a CPR EBD playfield I got from a guy. I am patiently awaiting Skateball, I really need a Paragon & would love a Fathom if those ever get re-re-run.
It's true you can't anticipate demand and the worst thing in the world is to be stuck with a pile of unsold inventory but honestly, with this crowd you could make double your SWAG estimate and still sell out. Unless you remade something truly obscure that is. Are you limited by production capacity? I used to have a problem with the Dunkin Donuts near me. They'd always run out of everything bagels. Always. I told the manager "Dude, order more." He obviously bumped up his order because now they don't run out.
I realize you're actually "making" stuff and are probably limited by the available inserts or whatever so it's not as simple as my bagel analogy...
As far as flippers go...well, first come first serve but it sucks to be the guy who is ready to do a playfield swap but can't because some other guy bought 5 to flip then resell for mega profit. Maybe you could limit people to 1 for the 1st two weeks or something to cut down on that practice? If a guy needs two he can have a buddy buy the other one.