There is a domain available in the .net zone that consists of two words and 15 letters, which perfectly represents the concept of my project. However, the domain used to belong to a Chinese online store with 3,000 pages in Google's index.

 I am hesitant to take this domain as I am unsure if it is the right choice. Another option is a single word, five-letter domain in the .biz zone that also reflects my idea, but may not be as aesthetically pleasing, for example, zcredit.biz. I am uncertain about what to do as I want to avoid making mistakes when choosing a domain for my large-scale project.

Based on my experience, domain history can be uncertain and problematic.
Therefore, it would be best for me to choose a new domain that has no prior history. If I find it difficult to come up with a suitable name, I should consider exploring other options.

I would not take a domain with a bad or inappropriate history. Bad karma has never helped anyone.

You've got a 15-letter, two-word.net domain that's a perfect match for your project, but it used to belong to some Chinese online store with a gazillion pages indexed in Google. Big whoop, right? That's not necessarily a bad thing, it just means the domain's got some history, some juice, some... okay, I'll stop.

Seriously though, having a domain with existing backlinks and a bit of authority can actually work in your favor. It's like buying a used car, it's already got some miles on it, but that doesn't mean it's gonna break down on you. Plus, you can always try to reach out to the previous owners and see if they'll let you keep the existing backlinks, or at least give you some insight into what worked for them.

Now, on the other hand, you've got this single-word.biz domain that's just... meh. I mean, zcredit.biz doesn't exactly roll off the tongue, does it? And let's be real,.biz is not exactly the most popular TLD out there. It's like the weird cousin of.com and.net. But hey, if you're dead set on using it, go for it. Just don't come crying to me when nobody can remember your domain name.
You're building a large-scale project, and you want to make sure you don't mess it up. I get it. But sometimes you just gotta take a leap of faith and go with your gut. The 15-letter.net domain might be a bit of a risk, but it's also a potential goldmine. And if you're really worried about the whole "previous owner" thing, just do your due diligence, check the domain's history, and make sure you're not inheriting any baggage.