More power to you.  I completely agree that the level at which the exchange operates needs to be addressed (but for now I also want to make sure the market forces operating here are sound)

As far as decentralization goes, this system would ideally not have to connect to a specific website or internet browser at all.  It may ultimately be managed through a special type of personal bitcoin wallet that can store this bitcoin, and keep account of it's credit/anti-credit pairs.

"Credit" itself may operate like a redemption token that is inversely proportional in price to bitcoin.  It entitles a user to more bitcoin as the price drops, and less bitcoin as the price rises.   Where it is "stored" is trickier to answer, because it doesn't necessarily have to exist at all.   When a "seller" posts an offer to lend bitcoin in exchange for credit, any person who already has credit (from previous sales) is entitled to take that bitcoin in exchange.  Credit is single-use however, and cannot be double spent.

"Anti-credit" binds the bitcoins within a specific order to an address, and does not give the owner permission to withdraw them unconditionally.  The process of taking possession of those BTC may ultimately look like some sort of multi-sig transaction which mandates that the BTC cannot be moved until the anti-credit is neutralized.  The higher the price goes, the less bitcoin a user has to 'sacrifice' in order to withdraw the rest.

Lots of technical details would be have to ironed out to make this provably work, but the result is an exchange system where trust is not an issue.

By accepting BTC, you are committed to selling some portion of the BTC back before it can leave and the rest is yours. 
By loaning BTC, you are given a proportional power to redeem this credit for BTC later