From the Credits page:
Notes About the Peoria.com Digital Economy »
" You can redeem the credits you've earned on The Exchange.
The Exchange may be closed from time to time. You can only redeem credits when The Exchange is open.
The prices of items on The Exchange fluctuate over time. Keep an eye out for when your credits are worth more on The Exchange!
There is a small daily "demurrage" fee of 1% or 20 credits, whichever is higher, that is applied to your balance at the end of each day if you have not posted to the forums for more than 3 days. This is to prevent stagnant accounts from messing up the Peoria.com economy calculations. Example: If you have 4,000 credits sitting in your account, and don't earn any additional credits and don't make any posts over the next 10 days, your balance will go down to 3,728 credits. Please Note: There is no fee applied if you post at least once every 3 day on the forums!"
Post, stay active, tip them or lose them.