Pet Store purchase I would likely quarrantine as many of the inverts/reptiles are stored in the area where they store things like the bark-type beddings which are usually infested with bugs.
However, buying from a dealer, if the spider had mites you would see them if you examine spider carefully....you would looking closely be able to see things or "dust" around the size of a pin-head MOVING - it's not like mites that would be found on a spider are microscopic afterall. Oftentimes, you would/should also see them crawling on the paper towels they were packed in inside the cup they were shipped in as all the jostling of the shipping would have knocked some off.
To me if the spider after being closely inspected looks clean and healthy, I don't see a need to quarantine. However, if anything looks or feels "off" then yes, I would quantine.
I receive a lot of spiders at my place, and I keep all transient and new arrivals in a separate room.
Risk of infection has no relation to someone being "reputable" or "ethical". Virtually all spiders that have had problems looked great at first (as they did to the dealer), then had issues. So no matter who the source is, at least basic separation for a couple months should be the norm. Or accept that you might do doing some serious sweating over your established animals, should a new arrival end up with a problem.
A similar issue came up a few years back with CB Crested Geckos. At the time I was keeping a larger number of them, all from the biggest-name dealer in the U.S. Well several started to get sick and eventually die. I performed a necropsy and through the histology we found an amoeba. Until that time, no one had discussed any real issues relating to CG health, and the breeders weren't talking either (fear of losing their reputation I suspect). But I still get emails all the time thanking me for the information to spot the problem and thus saving their CG (the info is posted in a few places, but if you want to read it, here's one place: http://www.ciliatus.com/content/index.php?option=com_content&task=view&id=42&Itemid=207
Bottom line is - anyone who imports has seen many cases of nematodes, and since they all do CB spiders, they too arrive with some risk of exposure. With more people talking about it, a treatment or infection outline will eventually be put together. Keep seeking it as an ethics issue, one that could hurt someone business, and it'll stay in the dark.
also, i thought vectoring was unknown for nematodes
I don't quarantine the T's, but I DO clean utensils between each T. when doing tank maintenance. Forceps especially, I clean w/ alcohol if I'm touching anything inside the tank like food boluses, waste, water dishes. I'm more worried about transmitting bacteria and other nasties from tank to tank than I am about things moving on their own from tank to tank. I also clean my hands w/ Avagard before/after maintenance. Tony