It probably has to do with color mix; I never buy from sites that have this kind of ad as I fee that they probably are getting rid of stuff no one wants. You are better off buying locally so you can see the flowers or trading with those who post that they want to trade.

Daylilies aren't bulbs. Are they talking about real lilies? This is the time of year you plant or move lily(lilium) bulbs.

the term naturalizing generally refers loosely to plants that are good at taking care of themselves. you just throw out a few bulbs at random and they seem to spread and fill out naturally. crocus, daffodils, some tulips, blue bells, lily of the valley, etc. mother nature seems to take care of them for us.

as for your specific questions:

perennial? yes. most anything that comes in bulb form is a perennial or at least a tender perennial. and in order to spread vigorously the natural way, it will take a few seasons to get going. however, even some non bulb, annual heavy reseeders can be considered naturalizing, like wildflowers.

hardy? yes. they can take care of themselves and stand pretty much whatever you can throw at them. but not to be confused with deer or rabbit resistant.

invasive> yeah i guess. in some cases. but it really depends on your personal interests. for instance, some people adore lily of the valley and think that it spreads beautifully. while i on the other hand think of it as a weed that i just cant seem to get rid of.

and it doesnt have anything to do with size, number of stems, flowers, etc. its all about how it behaves out in nature. but i agree with all of the above comments. i dont go for mixes, because they show you what you may be getting, when in reality you have no idea, and its probably just a bunch of clearance items that didnt sell. also, its true. daylilies are not bulbs. they have a "fleshy" root system that really looks quite creepy and alien at times. im not sure if it would be considered a rhizome or tubers or what, but its for sure not a bulb. as someone mentioned above, beware, what you are looking at may not be daylilies. i have also never heard the term naturalizing being applied to daylilies before. they do reproduce well, and are generally hardy. but if someone mentioned a naturalizing daylily to me, i would think of the common orange ditch lily...that has spread all across america of its own accord. but i wouldnt apply that term to the newer hybrids. hope that helps.