What are the legitimate Islamic regulations for earning profit in trade in general, and in the poultry and clothing trades in particular, and how can one deal with excessive bargaining on the part of buyers?

Bargaining in buying and selling is permissible by Sharia (definition). Evidence for this is the Prophet's (peace be upon him) sale of the mat and the cup. It is recommended to prioritize leniency in buying and selling, based on the hadith (definition): "May Allah have mercy on a servant who is lenient when he sells, lenient when he buys." As for the maximum limit for profit, there isn't one, unless the buyer is ignorant of the value and has trusted the seller and sought his advice, or if the deception becomes excessive, reaching one-third or more, under specific conditions.