Is it permissible to purchase an original copy of a medical reference book from a bookstore vendor who orders it from another city and increases its price, and is it permissible to circulate and sell scientific sources at exhibitions without verifying their original source?

There is no harm in requesting the original copy and purchasing it from the bookstore vendor, or in buying academic sources from international exhibitions, or from someone who previously bought them. One must not open the door to doubt, and consider what is bought as original, legitimate copies unless there is proof to the contrary. The default is that what is in a Muslim's possession is their property, and there is no suspicion in dealing with a discreet Muslim.