I would suggest keeping a really open mind about this, even though fake reviews do happen from time to time. to clear up the experiences I have had with fakes, once in awhile you will see an account that clearly has issues. The obvious ones are literally that - you might see a username jones1234 and they have a few reviews from jones2345. That is usually someone who is new and either attempts to create two accounts with little understanding or has a family member give them a few fakes. When they are caught, Fiverr is really good about either banning them outright or removing the reviews.
There are some other ways, but I really haven’t seen them as common. It is expensive for users to buy or trade reviews for one thing, and most sellers who want them the most couldn’t afford to do it. In years on Fiverr I’ve only seen one account where I found out the seller spent a lot of money to buy enough reviews to get a higher badge and that account eventually earned enough bad reviews to from real buyers to fail.
As far as sellers buying from other sellers, it might surprise you to know how much it happens but is actually legit. It’s very hard to tell from looking at the accounts. Some people are buyers for a long time and then become sellers. Many sellers run multiple businesses and buy services from sellers they know to help run their own businesses. The orders and reviews are legitimate. I have purchased from nearly a hundred different sellers myself. I’ve purchased writing gigs even though I have writing gigs because sometimes I need a topic that isn’t in my area of expertise. Most of them were people I didn’t know and I wanted to see if they might be able to provide me something inexpensive while they were new and cheap. Most of them did a pretty good job, though not all. I vet sellers carefully before I buy.
The rest of my purchases have been from people I do know and met on the forum. I buy regularly from some of them and some buy regularly from me, but every transaction is for a service that I really need. I’ve been tipped by forum users who bought my tip gig because I helped them here and that is still considered legit since tipping is allowed. So, I wouldn’t assume reviews are fake no matter who buys them. What usually tells you a review might be a fake is some kind of major red flag and it’s usually so few of them that it doesn’t make much difference. I would just steer clear of a seller anyway if you sense any sort of red flag. There are many to choose from! Overall, though, there are a lot of trustworthy sellers to be found. I hope that helps.