You are punished for changing the game time (no matter how much, it seems), but not the system time. Here's what happens:
If you change the time on your 3DS system, Animal Crossing seems to simply think that's the time it is now. Trees grow, events change, the only "bad" things that happen would have happened with the flow of time. Even going back in time via this method seems to work okay, even when going to a day previously played.
If you change the game time, using the option at the main menu of Animal Crossing, you will randomly trip for a while after changing the clock. The duration of tripping seems to last longer than the current play session, possibly the full day? But it eventually goes away. If, however, you were to change the game time this way before every play session you would trip randomly every single session, which is highly frustrating.
Also note that going backwards in time will spoil any turnips you had, since the game knows you went backwards, while it doesn't know you went forwards if you go by the system clock.
tl;dr: Change your 3DS's system time to time travel, do not mess with the in-game clock regularly or will will randomly trip.
When buying turnips, the price can be between 90 and 110 Bells.
When selling turnips, things get a bit more complicated. Looking at some data points, it seems that sell prices can vary within a huge range, but will follow certain trends. These trends, along with other tips on selling, can be found here.
Decreasing Pattern
In the decreasing price pattern, the prices will consistently decrease by a few bells each time, never increasing. With the decreasing pattern, the start price for the week will be between 99 and 50 bells. In this pattern, if the price doesn't increase by Thursday afternoon, you should sell immediately.
Big Spike Pattern
The big spike pattern features a decreasing pattern, but then there will be three increasing prices, with the third being the maximum for the week, followed by two decreasing prices that are still higher than average, followed by a continuation of the overall decreasing pattern. The maximum price in this pattern is always preceded by two increasing prices and always followed by two decreasing prices, so the maximum can't occur on Monday or Saturday.
Small Spike Pattern
The small spike pattern features a decreasing pattern, but then there will be four increasing prices, with the fourth being the maximum for the week, followed by a decreasing price that is higher than average, followed by a continuation of the overall decreasing pattern. The maximum price in this pattern is always preceded by three increasing prices and followed by one decreasing price, so the earliest that the maximum can occur is Tuesday afternoon and the latest is Saturday morning.
Random Price Pattern
The random price pattern features unpredictable prices in the range of about 50 to 200. The prices increase and decrease at random. The price seems to go over 110 at least twice a week in the random pattern.
By learning these trends, you can determine which day will have the highest sell prices for the week. Prices change twice a day. Once when the shop opens and again at noon. In order to properly watch for the increases described above, you will have to check every new price.
Best Answer
When you get access to Tortimer's Island, you're not allowed to bring your items from your town to the Island. The only way you can bring things back from the Island is to make use of Items Drop-Off Box next to the counter.
Anything you put in this box will appear in the basket back on your dock for you to pick up. It holds 40 items, and while it tells you to pick them up "as soon as possible", it's not yet known if there's an actual time limit to how long items will stay in there.
Also, make sure you put anything you want to keep in the box before you leave, as if you don't, anything in your pockets will automatically be sold to Leila, who will only give you 1/20th of the bell worth for your items.
Additionally note that the basket persists with your character; if you leave items in the basket in a friend's world then return to your own world the items will be in the basket in your town waiting for you.