If it's this one: http://www.pentaxforums.com/userreviews/sigma-28-70mm-f2-8-4-uc.html it's like a Pentax A lens so you have to set the focal length when you turn the camera on if shake reduction is enabled.
I suspect there are variants of the lens though in which case if it is a fully auto version I'll repeat the request: Can you check an image in camera to see what the image's info shows for the focal length. Take several at different zoom settings to check this. We need to work out where the 48 number is coming from.

What's gone wrong with it?

Yes, sniping is a good idea, save getting in a bidding war otherwise it might double in price!

Yep, the nearest would be f/1.7 or f/2.0. If it's truly a Pentax-A 50mm f/1.8 it would be an unrecorded priceless rarity worth thousands!
As to pricing, agree used eBay prices are a good guide.

Can you check an image in camera to see what the image's info shows for the focal length. We need to work out where the number is coming from.
Also is it this lens: http://www.pentaxforums.com/userreviews/sigma-28-70mm-f2-8-ex-dg.html

As the lens isn't Pentax, I guess the camera is being fed incorrect data from the lens. What I wrote above mainly relates to sorting out the lens type but ExifToolGUI will show other details which might give a clue. What does the Exif show for the lens model (if anything?) Usually its 'A Sigma Lens' without being specific.
Strange error though, perhaps someone knows the reason.
Does this error occur when checking direct out of camera images before processing, or after processing? Once in a processing program like Photoshop, the exif will have already been read by the program so may have changed the original data so no good checking that way!
ExifToolGUI won't change anything until you make it do so.
Also check the info in the camera to see which focal length is being set. The K-1 can be made to show this on the info screen. If it's showing 48mm but you're not setting 48mm the shake reduction will be set incorrectly too. I don't think that can be detail can be picked up that way on a K-5ii so check an incamera image for its focal length setting. If that's wrong, (48mm) you'll spot where the error is occurring, i.e. in camera. If it's right it could be in the writing of the metadata, or something to do with your photo processing program.

You don't seem to have placed your bid yet.

Why else would you want a grip?

Many a Pentax photographer wonders why their exif data fails to show the lens used.... sometimes!
In fact the problem isn't just one for Pentax users, looking at comments regarding images taken on other marques suggests the same problem arises. Yet sometimes the lens model does show up in the exif, what is going on?

So how can the lens model be sometimes included when viewing the exif in some images and not in others? The reason is that Pentax do not include the lens model in the exif data, ever. But it is included in the 'maker notes' that are also written into the images' metadata. It would be helpful if all cameras wrote the lens model detail to both the exif and maker notes so the problem being experienced by many wouldn't arise. Some brands may do this but it isn't an exif requirement.

What's more puzzling is that some images' exif include the lens model and some don't when someone else has used the same lens. The answer lies in the program that has been used to process the image, be it raw or jpeg. The Adobe programs, for instance, do copy the lens model from the maker notes to the exif, but it appears they only do this if the lens pre-dates the software. It's not that it cannot be done though so one has to presume it deliberately fails to make the transfer to persuade users to upgrade.

The easiest solution I've found which automates the process of transferring the lens model into the exif is a handy free tool named 'ExifToolGUI' which is a front end to a utility named 'Exif Tool'. Both programs need to be downloaded and placed in the same directory. The windows version will run directly whereas the Mac version needs an actual install.

ExifToolGUI can be downloaded from: http://u88.n24.queensu.ca/exiftool/forum/index.php?topic=2750.0
and its user guide from here: http://u88.n24.queensu.ca/~bogdan/
It looks complicated but don't be put off, the process to copy the lens model couldn't be simpler.

ExifTool can be downloaded from: http://www.sno.phy.queensu.ca/~phil/exiftool/
It could be used without the GUI but why make it more complex than it needs to be?!
ExifTool downloads with the filenme ExifTool(-k).exe, to use it remove the (-k) part.

So running ExifToolGUI and opening a directory will show the metadata for the images in the folder.
To add the lens model to the exif select the image or images you wish to transfer the lens model for and select the menu item Modify> Exif: LensInfo from Makernotes...
In not much more than a blink of the eye, all the files will have the lens model detail copied into the exif. By default it will back the files up, that option can be turned off it preferred (you may have a backup somewhere else so not need this.)
And that's it. It works for raw and jpegs and other types of raw files from other brands.

Those that use fully manual lenses which do not transmit anything due to having no electrical contacts could use the program to add the lens model to the exif manually by adding the LensModel to the Workspace which allows editing of fields.

Other useful functions of ExifToolGUI include the removal of GPS details from an image where privacy might be a concern and amending the date and time details of a batch of photos where you've forgotten to reset the date and time in camera.

Hope that helps solve what on the face of it seems a mystery.

Crackers to crack the nuts!