At a guess:

• People with steam accounts and VPNs in countries that steam doesn't operate in. Steam will block your "foreign" credit card as a fraud risk, but eBay dgaf cos it's the sellers problem if they get ripped off
• This is probably a pretty convenient way to send small amounts of money to people in a way that looks pretty legit. Arrange to buy some drugs off someone over telegram, they get you to buy a "steam card" from them, they send an envelope with a blank bit of cardboard and the drugs

There are two kind of buyers, this smart guy:

and grandpa, who bought it for his grandson billy:

Money laundering

As someone that used to buy these overpriced cards for PSN: A legit reason is that Sony didn't accept local credit cards, you needed to have an international one to buy games directly, so a lot of people just resold these gift cards to us poor mortals with our useless credit cards.

Either that or drug money laundering.

People trying to refund a tech support company after accidentally transferring too much money to themselves at the DOS prompt.

Quite possibly related to the Steam gift card scam https://help.steampowered.com/en/faqs/view/09A1-E754-1D86-DDC9

Often all the shops round here are sold out of steam gift cards for this reason. It’s sad, I spoke to a store worker about it, they explain to people they are getting scammed but they want the cards anyway.

Some people like to use a stored balance as a financial discipline tool. Don't put a "real" funding source on the account and then you can only spend the $100 you committed to, and not go whale-mad and drop $500 on premium currencies.

Man, I wish they still sold these, I would love to get one for a birthday.