Ultimately, it's going to vary on many more factors than the casino, but I think I can sort of answer the question
does this lack of reuse factor into the build quality of the cards?
First I should note that as I started Googling around for an answer to this, it is abundantly clear that there is not a definitive answer on this. This answer will assume you're talking about plastic playing cards since paper playing cards have such a short life span that they probably couldn't be resold.
There also is a lot of discrepancy on the length of time a casino even uses a pack of cards before retiring them. I have seen references that the Bellagio uses a deck of plastic playing cards for poker for 2 months, but this isn't official or even supported. That same page has a user saying they saw on a TV show that cards are used between 2 and 24 hours depending on what game it is and how they are shuffled, a claim that is impossible to check. This probably refers more to games like blackjack, where the house's money is on the line and they'd want to ensure that no money would be lost because players could identify cards.
The best information I could even find relating to casino cards was on the COPAG Cards website. In talking about use of their cards in casinos, they say
The primary use for 100% plastic playing cards in the casino is in the poker room. Since poker is a game where the players handle the cards extensively, plastic cards are preferred due to the durability and additional security that they provide over their paper counterparts. COPAG cards are used in casinos throughout the world and are dealt in many of the finest poker rooms across the United States.
Later in that same FAQ, there is this question and answer:
How long will COPAG cards last?
The longevity of your COPAG cards differs depending on how they are handled during play. Therefore, we can not give an accurate length of time that they will last. However, 100% plastic playing cards will last significantly longer than paper playing cards.
This is where I'm going to do some fuzzy logic. Given that on the same website they a.) talk about their cards being used for poker in casinos, and b.) they talk about the length their cards will last as if answering anyone that could buy their cards (and you can buy COPAG cards from their website), I would think that the used cards you buy from a casino are not qualitatively different than the cards you could buy new from a store, except that they've already been used.
I'm unsure how long a pack of plastic cards normally lasts since this also varies on how much it's used and how rough it's shuffled and myriad other factors. If you knew that, you could guess that any used pack you buy from a casino will have either 1.) months of almost constant play reduced from their life if they were used for poker, or 2.) a much smaller amount of time, potentially only a few hours of play, reduced from their life if they were used in games where the house's money was at stake.
Edit:
Figures that just after I post this, I noticed that there are separate tabs for purchasing on the COPAG website for retail and casinos, so some clarification is needed. On the retail purchasing tab, the plastic poker cards are measured at 63mm by 88mm. On the casino tab, the size of plastic poker cards is 63mm by 88mm. They are the same size.
Additionally, I would think there is no qualitative difference between the two for economic reasons. It wouldn't be cost efficient to have separate machines for different qualities of plastic for different cards, but changing the pattern on the back would be pretty simple.
