jimsen, I have found that certain topics on the Inca group take off and some of them go completely ignored. I think it is because of the format as an email listserv converted to a web front-end, as opposed to a forum.
Anyways, there was some bickering a while back about valuation of items and discussion of such. IIRC, someone listed an item, someone else questioned the asking price, there was some arguing about what the item should be worth, a few "an item is worth what someone is willing to pay for it" posts, and ultimately a lot of messages got deleted, and then some upset people later.
So it's possible that your inquiry went ignored because it got lost in other messages, or it's possible that people just steered clear of entering into that sort of discussion again.
I don't know much about the 420, but according to the Inca group (a 410 happens to be on the front page right now), it is "only" an 8-5/8" jointer. The 500-series are 10". It looks like the planer function on the 420 requires an additional gadget, apparently included in your item, but which looks a bit kludgy.
So since it does seem that you could get a 550 for not much more money (though in MI, and IF you are lucky), I'd probably pass on the 420.
I do know that I was offered a 410 or 420 for $250 Canadian a few months back. I passed because the seller didn't know much about it, couldn't send me pictures and was too far away for me just to take a peek.
Take that last price with a grain of salt, however. Inca prices are all over the board. You can find Inca auctions on Ebay that end up going well over $1000.
Regardless, whatever other Incas have sold for, it's tough to find an 8+" jointer for $500, and you could wait a long time for a better deal. I know - I monitor Inca listings on Craigslist and Kijiji in my area.