I just noticed something and I couldn't find mention of it in the guide.
Now I know you can sell practically everything by dragging it to the simoleons box in inventory, very handy if you don't want to bother with running around town, but I just found out you might not want to do that.
For example I had a butterfly worth 78 simoleons but when I sold it at the science facility they gave me 96, a bell pepper worth 48 sold for 52 and a seed worth 27 sold for 29 at the store. Same thing with books, you get more if you get out of the house and sell stuff at the appropriate place.
So if you want to maximize your profits it pays to make the trip.
Don't know if this is news to anyone, it was to me.
Also you can sell seeds at the store but they don't get included in the Sell all vegetables option, which is good. And I haven't found a place where you can sell the rocks and gems for more money.