How much control do merchant guilds have control over the economy during the medieval and renaissance?
Were merchants coerced into participating in the guild?
Did the guilds engage in price fixing and other non-competitive behaviors?
What are some notable examples of merchant guilds based on success and size?
What were the primary benefits and costs of joining a guild?
(My goal in asking the questions is to try to make an as-realistic-as-possible guild system in a novel I am working on)
Well, I'll provide an answer, however, I only know of the system as it existed in the Middle East/Central Asia/South Asia rather than in Europe so if your setting is European-centric it may not apply to you.
Guilds had an huge importance over the local and sometimes global economy. Firstly, they controlled not only the flow of trade but also the flow of money when a trader went from point A to point B. A merchant, say from Egypt, using the guild system which he had to belong to could bring a promissory note stating that at this point this person would pay him for the cost of the goods to say India. The merchant, using the promissory note, would be able to get his funds in India without any hassle and spend them while the other merchant could be assured that he would be paid. These networks of guilds and traders which belonged to them had access to local capital only by belonging to the guild.
Locally, to even establish a store or do a trade one had to join a guild. The guild controlled prices, who got to do the trade, and make sure that not only were the products were up to standard but the trade practices were on the level. These guilds would also be overseen by the market inspector who would ensure everyone trading at that particular market is on the level. Guilds were usually formed around anything that could be considered a trade, pottery, glass, spices, etc.
The primary costs were simply that the guild would ensure, if you were good, that there would always be a position for you in the market and would ensure that your son, if he was able enough, would be able to get a trade in either your guild or another one and be ensured a livelihood. Quite often guilds would also take care of older people in the guild through charity and ensure that your family would be taken care of.
Sources:
General:
Indian:
Safavid:
I actually answered a similar question about a month ago, although there's been a lot of recent interest in the topic here so I'm sure there's more room for additional discussion.