Page 2, Panel 2, Bubble 2: Change "had" to "has" as the line is spoken in the present tense.
Page 3, Panel 3, Bubble 1: Change "made" to "make" as while the potions are technically already "made" the shopkeeper is speaking in present tense as in "making the potions."
Page 3, Panel 3, Bubble 2: Change "is" to "are" as the shopkeeper is referring to Shin's potion(s) as multiple and thus it is plural.
Page 3, Panel 3, Bubble 3: Change "adventurers" into the contraction "adventurer's" so that the phrase makes sense so as to say "Every adventurer is always looking for your potions" or alternatively (and impractically), change the phrase to "Every one of those adventurers are always looking for your potions."
Page 10, Panel 6, Bubble 2: Add "are" into phrase to create "that are" to add the needed verb to the sentence.
Page 10, Panel 7, Bubble 1: You combined singular and plural here, thus you have two options (Yes both work): 1: remove the "s" from "potions" or 2: remove the "A"s in the sentence and add an "s" to "one." Both resolve the conflict and are grammatically correct, as well as not compromising the context of the scene.
Page 11, Panel 2, Bubble 1: the last bit "throw away the rubbish out!" is redundant. Remove either "away" or "out" to remove the redundancy.
Page 11, Panel 5, Bubble 2: Two problems, one panel. First is a spelling error: change "indipensable" to "indispensable." Second you have Two options again, similar reason to the last time. Either add "an" to say that "Potions are an indispensable..." or add an "s" at the end of the last word in the bubble. Take your pick, they both work.
Page 12, Panel 4, Bubble 1/2: Yes, this is a sort of "compound error" and yes it's another singular/plural conflict. You can fix it by changing one thing in either of the bubbles: Bubble 1 change: Make "wounds" singular, though I don't recommend it as the con-artistsalesman has multiple lacerations on his arm and thus this change alters the context, alternatively Bubble 2 change: Change "It's" to "They're" as, like I said, the posersalesman has multiple lacerations on his arm. As you've probably guessed, this is the change that I recommend, but as the person doing the work, the decision is yours to make.
Page 13, Panel 1, Bubble 2: Change "this isn't" to " these aren't" as the adventurer is most likely speaking about the potions as multiple individual things and not a collective product. Doesn't make sense otherwise, at least to me.
Page 18, Panel 3, Bubble 1: Change "customer" to "customers." This is an easy mistake. Though usually just caused by a typo due to rapid typing, it never hurts to make mention.
Page 22, Panel 2/3: Another "compound error" and the same issue as on page 12. It's also literally the same options for correction. Again, I differ to your judgement.
Page 23, Panel 4, Bubble 1: Change "group" into "group's" as this is in reference to the place that the cons hang out and essentially posses.
Page 25, Panel 2, Bubble 2: Two choices again. 1: Remove "as" from the sentence entirely. I see this a lot in scanlations and as far as I know it's incorrect. Alternatively you can change "called" to "known" and achieve similar results. As usual, it's up to you.
Page 26, Panel 1, Bubble 1: You've got a few options. You can change "actual" to "actuality" or just switch it out entirely with "reality." I know it's going to sound redundant, but this is your call.
Page 27, Panel 2, Bubble 2: Change "duties" to "duty." While the con is referring to multiple people, he's doing so in reference to a singular job in the group.
Page 35, Panel 1, Bubble 2: Add "a" so as to say "used for a bad cause..." or make "cause" plural.
Page 37, Panel 6, Bubble 1: The Skewers Chef doesn't have to say so much. Just remove "now" and "so" and the line is still in context and delivers the same message. It also reads better in general. This one is not entirely necessary, but it would help in creating a "clean read."
Page 38, Panel 2, Bubble 1: Change "a" to "the." Shin is speaking specifically about the rookie adventurer, not comparing him some random off screen.
Page 40, Panel 1, Bubble 2: This is another purely "read better" correction and by no means is it one that you have make. Remove "so" from the bubble.