I have a Trek 5000 where the Bontrager wheels were recalled for cracking, so I'd consider one Bontrager and one Shimano to be worth more than two Bontrager...

Obviously the seller was not overly anal. Or maybe anal about mis-matching

Mismatched wheels could mean the bike was involved in a crash or that the wheel wore out. Either way, I would look for other signs of damage like a bent fork, ripples in the steel (if a steel bike, especially at the head tube junction), or scrapes/bruises on components.

Then there's the question of what " non matching" means
I recently built new wheels for my new bike ( the ones it came with were kinda heavy)..
So now both have dt swiss 411 rims( although one is asymmetric), both have the same spokes with the same lacing pattern BUT the rear hub is Dt swiss 240 and the front hub is Hope.......it's what I had ... they don't match oh horrors . Oh yeah, the front also has red aluminum nipples...
​​​​​​

Knowing the bike is going to sell cheap, Sellers will often park miscellaneous parts on a bike for sale so as not to sell a better wheel set or other part. Wheels wear out/ break down from use in the normal course. So, simply not replacing a broken stock wheel but subbing something else is not that weird. Always inspect and ask questions when considering a used bike.

I would (and Have) check out the bike, give it a ride, give it a sprint, climb a short rise (if your area has one...). If all seems to operate well, and the frame & components show no damage.
If the bike rides as you expect. Then it's good.
Tires, bar tape, even saddle are expected consumables. I would always carry a chain checker to some used bike purchase scenario - if the chain is badly stretched, you may have to plan on replacing both the chain and cassette (worn cogs)...
It all dependenz...
If you're buying a pricey upper end used bike, you might want matchy matchy. If you're buying an older bike of any type, a mismatched wheelset is not a biggie, if the wheels checkout as good condition...
Ride On
Yuri

Please stop the madness. Non matching hubs and nipples!! Heretic!!!!

Just check the frame carefully. If the frame is good, and the price is right, you can always get a matching set of wheels. When I was a kid, my friend and I would build out BMX bikes with whatever wheels we could find. My absolute best BMX bike was a very heavy-framed Puch with God-knows-what kind of wheels.

Not here. Clearly we are talking about the bike not sporting the original set of wheels, wether they were identical or not. I think we all know that some wheelsets have different rims, spoke count, lacing, and other components.

I didn't notice it when test riding, but the previous owner of my Pinarello Nytro might have changed the rear wheel. It was after I took it home and went to swap for my fancier wheels that I realized the rear wheel had J-bend spokes while the front had straight pull. Looking more closely, the label said "Rapid Red 500" on the front but "Rapid Red 900" on the rear. I was kind of impressed they chose a wheel that looked so similar, but also kind of bugged now that I know it doesn't quite match.