Seeing the same issue with the guild merchants - also the 25% discount on Merchants star applies every time you upgrade one of those so the Traders and Caravaners just become free after enough purchases, lol. Also get a hard lock that I haven't found a way to get past when a "pick one of the two" option appears as the influence keeps going up but I can't interact with anything. Also couldn't seem to purchase the 9th star upgrade, but usually tried around the same time as the previous issue manifests so might just be the same issue (or a problem with gaining too much influence at once since I purchased enough merchant upgrades to have them granting many millions per second due to the upgrades being free).
Overall though, very fun and I appreciate the time waster. Thanks!