TF2 keys were sold for an all time high on the community market two days ago.

Probably people trying to transfer money to other account?

Region price differences, though there would have to be quite a bit of currency fluctuation for the keys to sell for that much more.

Maybe they're 2,5€ in Europe or 2£ in UK or something ? Anyone here to confirm such a price difference ?

Without knowing for sure, I'd guess it was caused by an issue with Steam's servers. I didn't see this spike, but I've seen other ones where the market gets screwed up such that people are temporarily unable to add new items to it, so the price goes up as the cheaper items get bought with no new listings to replace them.

As for why someone would buy them from the market for more than $2.50 instead of the store? I think ones from the store have a delay before they become tradable (a week, maybe?) while ones from the market can be traded right away, so someone buying them to trade for a game might have been willing to pay a premium instead of waiting.

Well, I do know that during that time, I could not post MY keys for sale. So "I" think that was the prob. Steam wouldnt let anyone post new keys and the demand in keys are still high. Not everyone wants to load the game up to get the keys from the manco store, and some don't know about it etc.

1Key on market here is 1.80€+ while 1 key on TF2 1.79€...

Since there were almost certainly cheaper keys on the marketplace, I would assume there was SOME kind of deal between the buyer and the seller. I'd gather it was some sort of convoluted deal involving ingame items as well.

There's someone running a ddos keeping a lot of users from using the market during a particular time of day, every day. It keeps some people from being able to add new items, but they can still buy. It's been going on for a week or so. I'm not saying this is the exact cause of what you're referring to.. but it could be. Pretty slimy way to regulate or force prices if you ask me.

Maybe some people just have no idea that they can buy keys from the in-game store. I had a friend who was buying Dota 2 Wraith Night Favor (an event points booster) for $1.05 on the market until I told him that he could get it off the Dota 2 store for $0.99.

there were someone who bought all keys below 2€ und put in the first 17 pages for that price

it is all about ~1 week of trading block for keys from Mann Co store (probably)

At that time TF2 inventory was broken for most of the time, you couldn't load it or trade. But prices on market are effect of broken sorting - lowest prices DIDN'T show up first. It was pretty random. I put 3 keys for little less than first page prices and ofc I couldn't see my offers.