7 days for their price adjustment policy, and 15 days for returns/exchanges.
If you are past the 7 days for the price adjustment, you can return AND rebuy your same 3DS if you aren't a dolt with your wording/irateness to the customer service clerk. The associates sometimes even encourage their customers to do it that way as there is nothing that can stop them otherwise, and it makes it easier for them so they don't have to do a package check.
Actually, depending on the item, if the clerk is following policy, they cannot allow you to return the system and then rebuy it past the 7-day price adjustment policy.
Select items that are returned (generally, electronics with internal memory) are not supposed to be resold (remember all those stories of parents buying electronics for their kids, only to find them pre-loaded with porn?). That's not to say it doesn't happen and that's not to say most clerks wouldn't just resell it to you. But, technically, by the letter of the policy, they wouldn't be allowed to resell you the same unit.
"The ultimate weakness of violence is that it is a descending spiral, begetting the very thing it seeks to destroy, instead of diminishing evil, it multiplies it."