Mark has hundreds of customers. If not more. Does bbts or sideshow email you every few weeks or months ti say "hey, this item you preordered is still preordered!"
Mark just invoiced and communicated with people who placed orders with him that he is ready to fulfill. No reason to worry. Mark will respond in a reasonable time when you place an order, when it's ready to ship and if you have any issues after it ships. Everything else in between, all these requests that he continuously check in with people, I think it's unreasonable. If you don't trust him to fulfill the order when you first place it, don't place it. Him sending a 100 emails to paranoid people that say "yes, the order you placed is indeed an order" doesn't really mean anything ti his ability or trustworthiness in fulfilling orders.
Mark is backed directly by XM, unlike many of the resellers in Singapore. That should be enough.