“How did people buy food from someone else before hashgohos”?
1. You relied on knowing the owner, including whom he/she followed in terms of halacha. Very few Jews lived in communities so large that everyone didn’t know everyone else (note: until the early 20th century, even the United States was still largely a nation of farmers).
2. Restaurants rarely existed before the end of the 19th century. Also there were very few processed foods, and almost all food was grown locally and processed by the buyer and their friends. While there are advantages of buying and processing only local grown foodstuffs, any local disruption would result in famine.
3. Products that had to shipped, such as wine, a hasghacha was needed, and wine often had several (e.g. the Rav in France where it was produces, the Rav in Prague who knew and trusted the Rav in France, and your local Rav who knew and trusted the Rav in Prague, etc.).