It's clear you're aware it's shady, because you wouldn't be asking if you were sure it was all fine. You know that you have to give a high rating and glowing review, or you won't get the same back, so it's not exactly a fair review.
But when it comes down to the actual rules, first up is Amazon. They don't like people reviewing things they're connected to (your own books, your family's books) the competition (any other books) or in exchange for rewards (like someone will review you if you review them). I personally don't review books on Amazon at all, because of their rules on competitors, but some authors do. For many products, a reward also includes a free product, but the rules do acknowledge that free review copies are a thing in the book world:
"Book authors and publishers may continue to provide free or discounted copies of their books to readers, as long as the author or publisher does not require a review in exchange or attempt to influence the review."
https://www.amazon.com/gp/help/customer/display.html?nodeId=201929730
There are also the FTC rules, which are going to apply if you're in the USA or not, because so many review sites are based there. My webhosting is also American, so being British doesn't mean I can do whatever I like on my blog. Here is a bit about the rules:
https://www.ftc.gov/tips-advice/bus...tcs-endorsement-guides-what-people-are-asking
The short version is that you need to disclose conflicts of interest. That means getting a free review copy should be stated in the review. Reviewing in exchange for a review should also be stated. These rules let you do anything legal, including review swaps or paying for reviews, as long as the review clearly states that you've done it.
How readers will judge those things is another matter. There isn't a rulebook for that, but if you're finding it shady, readers probably will too.
