Hey, just as a heads up, you need to be looking at drivetrain wear when you're buying used bikes OP. That blue one was being sold with a buildup of black crud on the derailleur jockey wheels. Not a deal breaker, and it does come from excess oil on the chain, but it's nasty and indicates that the shop hasn't done a thing to the bike other than put it out for sale, probably not even a test ride. The cassette also looked a bit worn, probably okay for a 1,000 kms or more, but still. When you buy a bike, you need to look for wear on consumables like the cassette and chain, and see how the seller is presenting it. Small things tell tales and you don't want to be blind to obvious flaws because you've mentally sold yourself on the bike.