[Pentaxxoid]

I purchased this DA 35 Macro used. The description did not mention any defects. Upon closer inspection I discovered scratches on one of the inside lenses, probably due to improper handling and cleaning of the lenses during disassembly. They are pretty difficult to photograph. What you can see is not on the front or rear lenses. They are on one of the lenses in the middle of the objective. I cannot see any obvious issues with the images the lens produces.

How much would these scratches affect the overall contrast/IQ?
Would you contact the seller and try to return/re-negotiate the price in such a situation?

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[ZombieArmy]

If you're happy with the results that's all that matters. However you should be paying an appropriate price for a damaged product and would try to get a partial refund.

[yucatanPentax]

I once bought a Limited lens from a Japanese dealer that was described as "like new." The price was not low. I wanted this particular Limited in near perfect or unused condition and paid for that condition.

As soon as it arrived, I saw there was a chip out of one of the center elements. My coworker had been a jeweler and had a loupe at his desk. We both looked at the lens with the loupe. Yep, it was a chip, like a windshield chip from a flying rock, out of an internal element. It was easily visible with the naked eye, just looking at it from the front. It was like a silvery glitter, plain as day near the center of the lens.

I sent it back. If the lens was "like new" I didn't see how the internal element could have been damaged in that way. Also, because it was so obvious, I didn't feel I could trust the supposed expert seller. The seller said he never noticed it.

In your case, it would all depend on what price you paid, what claims were made about the lens in the first place, and how you think the photos are turning out (without wasting many days of shooting to decided before addressing the problem).

Last weekend, I took apart and thoroughly cleaned one of my old lenses (roughly 50-60 years ago era). Even though I cleaned robustly, no scratches were evident either unassembled or assembled with the flashlight test. I'd say your lens had some pretty hard treatment to get those scratches.

[kypfer]

My only reservation about buying equipment with obvious apparent defect, even if it "doesn't affect the final image", is how much the re-sale value will be affected if I choose to part with it later.
Having a potentially expensive piece of kit that I can't eventually get rid of at a "sensible" price will usually make me have second thoughts.
Unless the asking price is money I'm prepared to write off in the future, or the fault is something I'm confident of fixing, I'll usually pass.
With something like a scratched/dusty lens, the only real way of knowing whether the resultant image is affected is to have a known good example available to compare it to.
If you've got a known good example you're unlikely to be interested in a second of the same type anyway

[Fogel70]

I think I would be more worried about the reason for the lens has been taken apart, than the scratches.
If it has been taken apart, is it still performing as it should?

[ProfessorBuzz]

That looks like fungus and scratches.

[mlag]

Inside is more likely fungus. Even if it are scratches, it was opened because of fungus, and badly cleaned (either insufficient leaving stripes, or to much brutal leaving scratches), so my guess bad cleaned & newgrown and more coming fungus is more likely ....