Does meat/poultry keep in the fridge for just a couple of days after purchase, or a couple of days after the sell-by/use-by date? I usually freeze all of my meat unless I plan to use it within one day, but I was just wondering whether I could keep some raw chicken in the fridge until Thursday (three days from now). I wanted to make sure it wouldn't go bad, so I did some searching and it seems that most sites recommend discarding raw poultry after just two days.

But the sell-by date on the package is August 6th, which is six days from today. The implication is that if I had boughten this chicken six days later, then it would somehow last in my fridge all the way until August 8th (6+2 days). 

Why does the usual food safety advice seem to be suggesting that raw poultry/meat will degrade much more slowly while on grocery store shelves? Are they kept colder there than in conventional refrigerators? Or will my chicken actually be safe to eat for another week?
The advice you see on sites is for poultry which doesn't come with an expiry date. It comes from the time when meat wasn't sold in individual containers with a date stamped on them. You buy poultry at the butcher, he wrapps it in wax paper, and you can keep that in the fridge for two days. 

If you buy your meat in the supermarket, it does have a date, and that's what you should go by. If it is a "use by" date, then it is clear - this is the last day on which it is safe to eat it. If it is a "sell by" date, you can again turn to the advice you found. It is calculated conservatively, as is usual in food safety, and gives you a margin which is safe even for meat sold after lying around for a few days at the butcher's. If you buy the meat before the "sell by" date, you "get" extra time to use it. 


  raw poultry/meat will degrade much more slowly while on grocery store shelves 


While the system above would work without the need for such a mechanism, there is indeed a difference which goes in that direction. The supermarket meat is packaged under special atmosphere, which is low in pathogens and also differs from normal air in a way which inhibits bacterial growth. It is not sterile, and it definitely goes bad. But still, when compared to the butcher's meat, its total lifetime from slaughter to expiry date is longer. 

This tidbit is not relevant to your practical question though, since this has already been accounted for in the choice of the "use by" or "sell by" date stamped on the package. I am simply sharing it for more background.