Currently not providing this as a service.

The idea is to stake items whose GE value is lower then their street value. How, you may ask?

Well, the two people staking can either trust each other and agree to sell the other person the item after the stake is done, or both can sell the items to a trusted middleman who will then give the items to the winner of the stake (either when both players tell the MM the winner or when one furnishes sufficient proof in the event of an argument).

For example...

Two players want to each stake (1) Royal Crown. The Royal Crown's current GE price is ~3.2m. This means that the middleman would need to have at least 6.4m gp, and both players would also need to have 3.2m gp in addition to their crowns in order to buy the second crown if they are the winner.

Items that this might work with include:
All rares (H'ween masks, santas, edibles, p'hats, disks, halfwines for the insane
Royal Crowns (note that these will inevitably crash like mints did)
Cheese (though these are slowly decreasing in price)
Smuggled dungeoneering items

Players could come to other agreements as well, such as buying each other's junk.

In the event that this becomes popular I will be able to provide MMing for cheese/crown/other small stakes until I build up my cashpile.