Have an opportunity to get ahold of what appears to be a Winchester Supergrade Grade III in 3006. Have handled the rifle and all is in pristine condition. Further considering rebarelling to 35 Whelen and shortening the forestock as tad so a barrel mounted sling swivel could be used instead.

Is the factory barrel saleable assuming 98% condition? What is a reasonable price for the barrel? Thanks.

You won't get much for a used "take off" barrel maybe $50-80$ hardly worth it. Maybe just send it to get re-bored, cheaper than a re barrel and I hear lots of positives from people that have.
I have a number of SuperGrades however I'm not sure what a Supergrade III is?
Other than looks I've never seen an advantage to barrel band sling attachments, I shoot some hard kicking rifles and never had the sling mount on the stock bite me but I tend to grip the rifle like I'm not going to let it fly out of my hands.

When I sell take-off barrels its more about helping out the other guy than salvaging much. Unfired Remington barrels move fast at 100 and slow at $125. You can take off a couple bucks for being used, less home gun mechanic friendly and being on the cheaper side of the medicine line, but it all comes down to who wants what that day.

Would you be cutting off just the black tip? That would leave the inletted swivel base, which wouldn't look that great at the end of the stock, and if you cut it off past the swivel base then you're at the checkering. That's not going to look good, so you could maybe put another tip on, but I'm staring at my own Supergrade and not really seeing a way of shortening the fore-end without looking like a beaver chewed it off.

The 35 Whelen part is interesting, but I'd leave the stock alone.

Always harder to sell a barrel with an extractor cut. Harder to mount and headspace on a new action (my opinion)

While I like the 35 Whelen I would leave the stock alone.