You could have each logo price decrease by $2 per day it remains unsold. The starting price would follow the current rules, increasing $2 for every sale. Each

You could have each logo price decrease by $2 per day it remains unsold. The starting price would follow the current rules, increasing $2 for every sale. Each logo should naturally go for a fair 'market' price (for that logo), and presumably will eventually be sold by the time it reaches $2. It prevents the service usage from leveling off as the price goes too high: at some point, it is cheaper to pay for a custom logo designed to spec. Plus it increases traction, especially for designers that need logos for clients: now there's incentive to come visit the page occasionally even if you don't currently need a logo, because there might be a gem sitting there for a good price.