The worst thing the plants could bring is Algae or unwanted snails. Ive never quarantined plants before but if you do I would be sure to keep the water clean and have good like to keep them alive while you inspect the plants for anything.
Some plants can actually carry diseases into a tank, so it is a good idea to quarantine for two weeks. I don't, because I absolutely trust my shop (a local community of hobbyists, experts and breeders), but if I got them from a petshop, I would.
As for Aquabid...it depends on how reputable the seller is, I suppose, and if he grows his plants in plant-only or fish-in tanks.
I got my plants in a tube with agar jelly and fertalizer so I trusted them enough to not need a quarantine. If they come from a tank I would suggest like bombalurina said to quarantine them for at least 2 weeks for the parasites to die off.