Finley in theory is good. He could be better.
Tying into his own story, where he is “roaming in search of rare items”, he could be found in the wild instead of a set timer on a Saturday. He could even be tied to the Bounty system that already exists, as a Weekly Bounty, in the form of a rare mob, that can be found in any area. You would find that mob, kill it and then Finley would appear to investigate the loot, since that’s what does. Upon going away from the area he could disappear then. This keep him as once-in-a-week vendor but you don’t have to do it on a time you might not be able to find him(Set as Saturday Morning). You could do it on your own time.
Secondly, he could sell rare upgrade materials for big QoL improvements for Sacrament which means he would be “necessary” and people would actively try to find him.
Finally, he could also sell a wide array of upgrade materials for rerolling stats and/or affixes on items.