A 52V 30AH battery is a large, heavy battery for an ebike, and not a common size. This much weight should be carried in the triangle for balance, and it needs to fit in the particular frame. As was pointed out, it needs to deliver the current that the controller requires without exceeding the BMS ratings. The cells and interconnects must deliver this current without heating up. The pack build quality must be high avoid having shorts and fires. Many practices for low cost compact battery building, such as gluing cells together, don't produce the safest packs. The battery is perhaps the most important component in an ebike build, but the quality is hidden and you can't easily inspect it.
Shipping lithium batteries is difficult, and you cannot return a damaged battery. It is best to have a local source or service organization in case you need service. And batteries do occasionally need service.
You also should use a quality charger. This is also a safety item. It should be certified electrically safe from a Nationally Recognized Testing Lab. Most are not. These safety ratings are often ignored by low cost manufacturers.
How much of a risk taker are you? One friend that bought an Ali Express battery had it burn his house. The impact of a house fire is pretty major.
Focusing too much on price can result in marginal quality and safety. The cheapest solution to transportation may not be an ebike at all, used scooters are hard to beat. A quality safe ebike is not something to expect at the lowest price point. To get to that price they have to cut corners somewhere - in the bike, the motor, the battery, the controller or in the QA - and probably in all of them.
Take care and be Safe.
