I can't provide historical sources whether merchants are actually disliked or not in pre-modern era, but as for why they might be disliked, I'd refer to the work in which Barrington Moore is famous for: Social Origins of Dictatorship and Democracy.

His main argument basically goes by: middle-class (which consisted of merchants) threatened the structure of medieval economy.

From trading goods, merchants were able to accumulate profit and circulate money. They didn't have to work under the serfs like the peasants did; they were, in some way and another, "detached" from the bound that binded the ordinary with their lords. They were able to gain wealth relatively faster than the peasants. Also, to put it in cultural terms, they were the class which was able to replicate the culture of the aristocratic elites. In many societies working under the medieval structure of economy, it was the merchants who simulated and copied the leisure time activities and arts enjoyed by the elites (e.g. specific ornate from vases, or paintings). Not a Western society, but this was particularly true in Ottoman Turkish (not mentioned by Moore, but I noticed a similar pattern): when the elites enjoy muqamat, or certain artsy sculpture such as vases from far-away kingdoms, it was the merchants who would get an imitation of it. Easily said, merchants had the material and cultural advantage.

Thus, the development of the merchant class, according to Moore, would lead to the weakening of landed aristocracy and leaning more to the commercial agriculture. This of course would pose a threat for medieval landlords as not only their powers was in peril, but the merchants could also pose a danger to their "pride" as the cultural elites. Again, not a Western society, but this example was absolutely true for the Japanese samurai landlords (again, not mentioned in Moore's book): the merchant class got too rich to the exent that the richest had their houses more decorated in luxuries than your average samurai bureaucracy workers. This made the shogun had to ban their public display of luxury as it caused jealousy.

Finally, it is the merchants who gives the biggest contribution to industrialization.

Moore's thesis ends with two possible outcomes in this development of merchant class: first, democracy, as observed in England, USA, and France, if the middle-class managed to develop a strong, independent class which completely severed the old feudal tie (as stated by his own words: "No bourgeoisie, no democracy"). Second, fascism/communism, as observed in Japan and China, if the old aristocracy (or the peasants, as with case of China) managed to form an alliance with the new nouveau riches before the aristocracy was completely destroyed.