I've got 3 Sebenzas and an Umm and only one did I pay $300 or above for and thats because its a P model. You just have to know exactly what you want, be patient and keep the cash on hand for when the opportunity presents itself.
Edit: Out of the three knives that suddenly became available to you my vote goes for the SNG. I have a ZT 551 and its one hell of a knife but something about the SNG just really does something for me. Maybe its the one piece of G10 that makes the scale and backspacer, the high hardness of the S30v, the opening hole, or the many different configurations that this model is available in. That said I've never owned an SNG so its all just from what I've seen of them and the fact that for the last month I cant get them out of my mind. Anyway, good luck on your decision.