Could you explain them? I always tried to listen to Joan when I (rarely) saw her but I never could quite understand...
Ok I don't know if red turnips are in the game or not (and btw I'm just going off how turnips work ni CF/WW they might have changed in this game so i donno)
but red turnip seeds can be bought for 1,000 bells - they need to be planted and then watered every day, then after a week (i think it's a week) it can be dug up and sold to nook for 16,000 bells (that's a 15,000 bell profit) - obv they get sold to the alpaccas in NL and like i said i donno if red turnips are in this game
Anyway, in CF, Joan would sell white turnips for 90-110 bells (price would change each weak) I'd only buy then if they were under 100 bells. Anyway, you buy them in stacks of 10, and then you have a week (so until the next sunday) to spend them or else they spoil and are useless (though that's not a problem since you have TBT and there was a turnip price topic so it was easy to gain profit) - anyway, Nook buys white turnips for a different amount every day (he changes his price at noon and midnight [12PM/AM]) he doens't buy them on sunday - his prices range dramatically (i think the lowest recorded it low 40's and the highest is like 600) so there's a huge range between Nook, and the chances are more likely to have prices higher than you bought than less, so you gain profit really easy.
So you go to Nook twice a day, after noon and after midnight, and see if he's buying turnips for a higher price then you bought them for. some weeks he might have low buying prices (50-120) then another week he might have high prices (50-300) - there are some people who claim you can find trends in his buying pattern and it is possible to predict which days he will have higher turnip prices. But I haven't been able to do this. So I donno
Anyway, pretty much just go to Nooks every day and see if he's buying them for a higher price than you bought them and you gain profit. Like I said it is risky since he might have a low price that week, but like I said, there's other people on TBT so just ask around for turnip prices (and I can gaurentee [that words spelled wrong and i cbf to fix it up ok] someone will have higher prices than you) so yeah.
Oh and time travelling causes turnips to spoil and effects the prices
anyway, one thing people need to know about me is I suck at explaining things, so here's a link which explain it (yes I know its a wiki but I have checked it out and it seems pretty valid)
http://accf.wikispaces.com/Turnips
hopefully i helped