Is the ability to spend coins for something coming? Currently there is no liquidity, I know, but you could always use the minimum marker price or the last month average if sufficient proportion of users were active on exchange..

I think it would improve the liquidity a lot and even if someone would choose to cheat, he'd be paying the fee. You could always put top exchange for cases when the price will go higher, you'll use the limit. Example: you could say that one EAR is not worth to your mining company for more than .01 BTC and let users shop for this price at most (so if I'll cheat and drive the price to 10 BTC, my rig upgrade would still cost as it would at .01 BTC, but when the price falls down to .001 BTC, I'll pay ten times more EAR.

You'd be required to watch markets and make some tuning, but it's worth the time!