first of he is naive as Fk. Having that as his only party ever and secluded from all other adventures.
It was a guild recommend party, which the person who told him explicitly how good they are and how bad he is.
That said the guild here is corrupted as Fk. To the point they deliberatly recruit crooks and push out good hearted adventures.
Civilians already distance them self from adventurers as 90% are crooks.
Also as a child he grow up being said he has no talent. Being stupid he also trained excessive amount on something he has no talent for (heal) till the point he is good at it.
The items he left behind were bought as aloan from the party, meanwhile the party leader and his second embezzled most of his reward, so that why he thought all rewards are 80% less that they actualy are.