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Someone used the market to send money between accounts, $73.30, by selling a cheap item for that much. Not ideal since so much goes to fees, but it's relatively safe.

Pity there's no way to hide/remove it from the graph though, it absolutely destroys the display.

Its a way to transfer steam wallet from one user to another

Even though the 15% is high , some users still want to do it

So they put something for sale , for e.g. emoticon or even Dota 2 .

Telling the buyer the exact price they put , the buyer finds and buy , and the seller is giving him the key/gift :)

Probably someone trading Steam wallet between one account to another.

Account 1 has $XX.xx spare wallet and wants to give it to account 2 (for trade? The only other reason I can think of is below). Account 2 lists something that is normally cheap in the market way above what the rest of the market will pay and then account 1 buys that item for the high price and transfers wallet over to account 2 (with Valve taking a nice little tip on the side).

The only reason I can think is someone setting up accounts to idle TF2. Makes new account, adds some wallet, buys TF2 item and gets premium (sell the item to get some of the money back). The owner then sells DoTA 2 / other practically worthless item from main to the idle account and gets pretty much all the wallet over on their main. Sell a few of the items from idling and you've recovered the 15% tip that Valve took when you transferred your wallet across. Any extra items from idling a then profit.

I don't know why the item OP mentioned went for such a high amount, but probably a trade agreement (bit of a risky trade - but no more risky than PayPal and it doesn't require both people to have PayPal.)

That price :P