I use a flea market app called "mercari".

I think it's a quite a popular app in Japan.

For example, I often buy books written by popular YouTubers, and I sell them as soon as I finish reading them.

It's not like I can sell books at the same price as I buy them for, but I guess it's better than keeping them at home.

Actually, I got my hopes up too high for the book and after read it, I was like "ummm, this was not what I wanted."

Just for the record, I'm a big fan of his channel and I like his videos, just for the record.

I read a book written by Tarosac.

I use a flea market app "mcalled "Mercari".

The technical rule for quotes next to commas and periods is that the comma/period goes inside the quote. I think it's a dumb rule and tend to intentionally break it when there's room for ambiguity, like there is here (i.e. if you typed "Mercari." then it would be ambiguous whether the name was "Mercari" or "Mercari.").

The other day, I sold a book written by athe popular YouTuber "Tarosac".

If you want to use "a", you should say "a popular YouTuber named "Tarosac"". The reason is that with "a", the impression is that you're introducing something that was not already in our attention, so you describe the YouTuber as being "named "Tarosac"". However, with "the", the impression is that you're referencing something that just exists (maybe we didn't known about it, but that's not the focus), so you can call the YouTuber straight by their name.

As expectedNo wonder, he is a popular YouTuber.

さすが can mean "as expected," but here it carries the reverse meaning. From the fact that he is a popular YouTuber, it is expected that the book would sell quickly. If you write it with "As expected", you're saying that, from the fact that the book sold quickly, it's expected that he is a popular YouTuber.

It's not like I can sell books the same price as I buy them at, but I guess it's better than keeping them at home.

Actually, I got my hopes up too high for the book and after I read it I was like "ummm, this was not what I wanted."

I read a book written by Tarosac.

For example, I oftenI usually buy books written by popular YouTubers and I sell them as soon as I finish reading them.

I think it makes more sense for you to just say usually instead of for example given your previous sentence.

As expected, hHe is a popular YouTuber so it makes sense why.

I assume you are trying to say that it should’ve been expected since he is a popular YouTuber? If so, I think that saying the sentence makes more sense and sounds more natural.

It's not like I can sell books for the same price as I buy,ought them, but I guess it's better than keeping them home.

Actually, I got my hopes up too high for the book andby Tarosac but after read it I was like "ummm, this was not what I wanted."

For the record, I'm a big fan of his channel and I like his videos, just for the record.

I use a flea market app called "mercari".

The other day, I sold a book written by a popular YouTuber named "Tarosac".

Or,
the popular YouTuber "Tarosac."

I was surprised because it sold in (just/only) one day.

As expected, hHe is a popular YouTuber after all.

「さすが」の英訳は文脈によります。この場合、こちらの方が自然な表現かもしれません。

It's not like I can sell books at the same price as I buythat I bought them at, but I guess it's better than keeping them at home.

I sold a book written by Tarosac.

I use a flea market app "mcalled "Mercari".

it's actually capitalized in English

I think we would more correctly say "Marketplace app", but I get what you're saying here.

I think it's a quite popular app in Japan and I often sell my books there.

The topic of the previous sentence is the same so this is fine.

I often sell books there.

This should probably not also start with "I" but it's so simple it's hard to remove so I put it in the above sentence.

For example, I oftenregularly buy books written by popular YouTubers and I sell them as soon as I finish reading themright after finishing.

"I often" again

Trying to increase the sentence variety.

I was surprised because it sold in just one day.

added emphasis because you're surprised.

AsMay I should have expected, it; he is a popular YouTuber.

if you're surprised why is this excepted? We don't really say like 流石 in English in this way.

It's not like I can sell books the same price as I buy, but I guess it's better than keeping them at home.

Actually, I got my hopes up too high for the book and after reading it I was like "ummm, this was not what I wanted."

For the record, I'm a big fan of his channel and I like his videos, just for the record.