Why doesn't the apothecary want to sell poison to Romeo?
It is against the law to sell such strong poisons to just anyone who walks in the door. The reason for this is that when the practice was legal, many people used these quick poisons to exact revenge on their enemies. To protect the citizens of Mantua and other nearby cities, the law was passed that apothecaries must not sell the strong, quick-acting poisons to just anyone off the streets. The buyer must have the proper government papers to show it is an approved sale.
Romeo does not have these papers, but he does have gold. The apothecary knows it is against the law, but the gold is too tempting. He tells Romeo, "My poverty and not my will consents." Romeo responds, "I pay thy poverty and not thy will."
In Mantua, it is illegal to sell the kind of poison that Romeo wants. He wants a strong, quick-acting poison that will kill him immediately. However, because the apothecary is poor and really needs the money, Romeo is able to convince him to sell it. Romeo pays him with, "Here is your gold,/Worse poison to men's souls", indicating that gold has caused more harm in the world than the little dram of poison that he is about to take. Money corrupts and can change people (thus "poisoning" their lives), while the poison he bought will bring Romeo together with Juliet in death.