It doesn't matter whether they can onsell the product or not. What matters is, they've allocated their batch based on a number you're included in. Cancelling is worse if the preorder deadline has been met with their distributor, and it's far worse if the item is in stock. And don't even dare think about cancelling preowned from Amiami or Mandarake unless you want your ass banned, since the item is in stock and likely to be prepared at any given point from the time of your placed order.
Allowing cancellations may attract more customers (see: HLJ), but Amiami attracts customers by having the lower discounts. In Japan, the rule is the same: Japanese use Amazon Japan since shipping is free and cancellations are allowed, while Amiami's shipping is not free and they don't allow cancellations. There is actually very recent talk of Amazon Japan refusing cancellations on games and figures because it's messing with their inventory. If Amiami allowed cancellations freely at this point, it would just do more harm than good to their entire store, not to mention their credibility. These kinds of stores have to stand their ground.
Of course if the purchase to cancel ratio is high, they're going to allow cancellations, because you're still a frequent customer throwing money at them. It's trade 101.
