Does anybody have a clue why this would cost 999?

No idea, but it does come with free shipping. Possibly they meant .99

It's an error even .99 USD is high for a single pen

You can get a pack of 25 of these exact pens for the about the same price

It could be because it’s out of stock and removing a product from an online store completely hurts a sellers search rankings. Something to do with search engine optimization. They want to keep it active but don’t want you buying it so they set the price crazy high.

For a millionaire who always wants the most expensive pen, car or other stuff :D

Two reasons;

1. Human error: data entry mistake or listing mismatch.

2. Money laundering: Offer COD for such a cheap item. Buy the item from another account. Collect cash. Deposit the cash in the business account and pay tax on it at the end of the year and black becomes white.

It's to make people send pics like this and give them free promotion 

It's filled with snake venom.

That's one of those military budget pens

Pen smuggling

They do it for attention so when people post ads promoting the sale of this singular .50 cent blue pen they can get engagement in the form of “Does anybody have a clue why this would cost 999?”