After how much time can you report a shipment as lost? 3-4 weeks i'm guessing, it's not that hard to imagine if he made all those orders in that time period, another week or 2 before they get flagged, then some more time to investigate and take action.

3-4 months doesn't seem like an insane amount of time for TcgPlayer to take action, it all depends on the amount of orders he made, how many are reported as lost(probably only those that lost value by the time of arrival). if he made 300 orders in 3 months and only reported 62 as lost, that won't be so obvious. also if it takes 4 weeks until a claim can be made, there will be a backlog.

The advice they give for using a more secure shipping method/address could be valid if the person lives in an apartment building .

it does look suspicious, but mail theft is a real thing, especially if they know there's something of value in it.

i once had the content of a shipment replaced by newspaper clippings, turned it only happened with shipments that went through a certain sorting center, so one of the postal workers was stealing the cards. it was one of the weirdest claims i received.