Turnips so the creators could make a pun and call it the. STALK. market.
*badum tissh*
As for patterns, there are 4:
- Decreasing pattern (where the turnips will start off lower than 100 bells each and decrease over the week. You will lose on turnips if you do not find another town with a good price)
- Big spike pattern (where it begins to decrease at the beginning of the week, exactly the same as the decreasing pattern, but on Thursday it will suddenly rise, and it was peak at the 3rd rise and that is when you should sell)
- Small spike pattern (exactly the same pattern as big spike, but there will be a small increase on Thursday and its price will peak on the 4th rise and that is when you should sell)
- Random/ Rollercoaster pattern (completely and utterly random, meaning you should check every day before and after noon)
Time travel wise, basically the above advice from Bulbashoe is 100% correct. I've heard that if you travel backwards, e.g. you want to turn your clock back 1 hour when the clocks change in winter, you should do that on your 3DS' clock, and not your ACNL clock. Then, do not play ACNL until the time has passed, e.g. you turned your clock back from 5pm to 4pm. You do not play until 5pm has passed on your current clock. Then, your turnips won't spoil. I haven't tested this out myself, but that is what I've read.
Thonky guide on Stalk Market