According to their website, they only release some tickets online, so "Sold Out" doesn’t actually mean you can’t go, only that the tickets released online have been sold out.
From the website they suggest their tickets may be bought out by scalpers:
Resellers of our entrance tickets are active on the internet. They buy
up our entrance tickets online, and attempt to sell them at a large
profit, often for double the price. This is not permitted. Do not buy
tickets from these organisations; you may be refused entry.
… and in the FAQ section…
I want to make a timed reservation using the online reservation system
on the website, but I see that very few times are left available. Is
the service really fully booked, or is there another way to make a
reservation? On the website you can see exactly what times are still
available. If you still want to come at a particular time for which no
more online tickets are available, then you can always buy tickets at
the Anne Frank House entry desk. However, you may have to wait in the
queue.
The place is almost always busy as noted by many reviews online and from my own experience as well.
Here are some tips (some are common sense):
Finally if you do see a line, how long is the wait?
- Estimating the wait
How can you gauge your time? If the line is an L shape, ending just
along the Westerkerk, that’s about an hour. If the line is an L shape
but ends past the Westerkerk and at the Keizergracht canal, that’s two
hours. If the line is a Z and has now turned at the Keizergracht canal
and is running behind the Westerkerk, that’s three hours. Any longer,
well, you do the math.
Credit:stackoverflow.com‘