Originally posted by seventysixersfan Yep, got it. But as I said, I think Canon and Nikon versions of the non-OS lens may be priced a little lower on the used marketplace because those shooters would prefer to have image stabilization built-in to the lens. Since this one doesn't have it, the price has to be lower to attract buyers. The Pentax version of the lens doesn't have to account for this omission.
I think there's two big reasons the Sigma 50-150mm f/2.8 and the Sigma 100-300mm f/4 demand a premium in K-mount but sell for much less in Canon and Nikon mounts. You already pointed out the first reason, which is the lack of stabilization in the lens, which is a real bummer for a camera without in-body IS.
The other big reason is that there is no shortage of great telephoto lenses for Canon and Nikon users to choose from. On the contrary, there is an abundance of choices in telephotos, and most of the best ones are made by the camera manufacturers themselves. Pentax users, on the other hand, face a much different situation. We can't even count on the 3rd party lens makers to always support us.
Witness the new
Tamron AF 70-300mm f/4-5.6 SP Di VC USD, which is very sharp and is probably the best consumer 300mm zoom lens out there. But it is not offered in Pentax mount, and when questioned Tamron says they have no plans to release it for Pentax. So when a company like Sigma comes out with a great telephoto lens and offers it in all the various mounts, it will be received with much more enthusiasm by Pentax users. (If Tamron ever did release that lens in Pentax mount, I'd buy it in a second.)
And in the case of the 50-150mm specifically, there's really only one alternative out there for Pentax users, and that's the Pentax 50-135mm. But that lens is problematic in a couple big ways. It is very expensive new, but on the used market there is always concern about the dreaded SDM failure. And on top of that, the AF is not very fast.
So if I were in the market for such a lens, I would jump at the chance to buy a 50-150mm. I would certainly pay more for a used Sigma 50-150mm than I would for a used Pentax 50-135mm. And I think the 50-135mm goes for between $700 and $800 used. So I think that anybody who got one of these (basically new) 50-150mm's for $500 got a screaming deal. I'm not surprised at all that they sold out as fast as they did. I'm just surprised that Sigma listed them so cheaply.