I'd say go for it, offer his $450 cash, if you got cash in your hand, he'll probably bite.
the Tenba case alone I've seen sell on ebay for $100 and up. If the lens is a Caltar II-N 150mm, its a re-branded APO-Sironar-N lens, $200-$300 or so is the general selling point for these in good condition. Camera, maybe $75-200 depending on model, and condition. 4x5 holders are about $5-10/each depending on age/condition, also on how much buyer is willing to pay.
if he won't budge from $499 on the price, see if he'll throw in a box or two of film(10sht) to get you started. Otherwise, I'd offer $400(cause I'm a business-minded kinda guy, and the dealer probably paid $150-200 for the whole kit from someone wanting a D300s
)
but this would be a nice kit to get started in 4x5 with, if you're just getting started or want a rail camera to work with. They're nice cameras, both the Toyo's and the Omegas.
-Dan