On a normal credit card, not a debit card, is it legal for me to purchase something for 6 dollars and have them ring it up as $56 and give me $50?

Most normal Credit Cards let you get cash advances, so just buy your 6 dollar item, then go to the ATM and get 50.00 out.
Yes, you need a PIN number to do this. You sat that up when you got the card.
I notice though, that some cards of mine (Discover, Cap One I think) do give me the option of a small cash advance at Wal-Mart.

It's not illegal, but most merchants won't do it, since they are in effect throwing away money, as they need to pay a % of each purchase.

i dont think thats its illegal...if your card allows you to get cash advances than it should be ok...but i think the interest is typically higher for cash advances than for purchases...not totally sure though

Credit card companies charge a percentage to the store when you pay so that's why the shops don't let you can't get cash back. Also you usually get charged a higher interest rate for cash advances which is the card companies don't let ya.