Expecting people to delete their own pictures 
won't work.  Too many lazy people out there who 
won't care beyond the free advertising aspect of 
the site (and plenty others who will just plain 
forget).

Instead, put an automatic expiration on every cel 
listed.  Say....2 weeks.  After that, the system 
automatically deletes the cel.  This would keep 
listings relatively current and would encourage 
the "short and sweet" descriptions you want.  ;-) 
 And it should discourage general abuse of the 
system.

You could make the time limit longer (maybe a 
month) depending on how laid back you want to be.

Heck, beyond a tiny (less than 10KB) image of the 
cel for sale, the site doesn't need to contain any 
other info as long as their is a link to the 
seller's home site with more info.  I see this 
sale site as just a pointer to get folks started. 
 (Good idea, though!)

JP