I work in the furniture industry and can answer some questions here.
Yes, buying any kind of upholstered seating used is a bit like rolling the dice - you just never know what the former owners were doing on it. You put the odds in your favor by looking at something that's leather as it's potentially easier to clean and sterilize. Your best bet though is to make sure you're buying from someone who is wealthy and stable. It's not foolproof but it puts the odds in your favor that the furniture wasn't used as a prop for someone's deviant fetish.
If I was shopping for a sofa on CL I would always ask for the manufacturers name and then check online for prices on comparable models. If new sofas by that manufacturer retail for anything less than $3000 (that's actually not that much in the overall scope of things) then avoid it - end of story. If the original owner spent at least that much it's the best indication you can have that they didn't abuse it.
Same goes for pullouts and recliners although I'd adjust the number to $4000 and $1000 respectively. This really limits your choices on CL but also assures that you'll get a quality piece in addition to limiting the chance of any grossness.
As to the second question, any new sofa that you pay less than $1000 for is crap. $2000 get you into the almost-decent category. Don't take anything you read about "clearance" sales at face value. For that matter, the same goes for "going out of business" sales. There are companies whose entire business model is to lease spaces, open a store and close it as soon as legally possible just so they can conduct a big long GOB sale. They buy the cheapest furniture known to man - I mean absolute shit - mark the price way up and then tell you you're getting a big discount off of regular price. And of course the public eats this up because they think someone has lost their business and opportunity is to be had. Don't believe it!