Search through other people's newspapers for construction equipment like saws and dynamite and buy them in bulk (for example, five items for 400 coins). Then sell them individually for between 180 and 220 coins apiece. On the very last item you list place an advertisement on it so people can see ALL your items when they come to the store.
Buying Missing Crops:
If you are missing some crops simply go to a friends farm and find their roadside shop and tap on it and see what they are selling. Or if your level 7+ open your newspaper and see whats being sold.
Provided you have an available slot you can sell stuff at the roadside shop as often as you want. You can only get a free advertiment in the newspaper every 30 mins.