Put all of your item-types in a variable array. Do a trigger that sets a random number between 1 and the number of items you have (ex. 50) . When the number matches with the item-type in your array, add that item to the shop. Repeat for the number of items you have.
Finally finding about some of the bots that are flooding the users online - bytespider apparently is a huge offender here - ignores robots.txt and comes in from a ton of different IPs
Ghan has said he has fixed this. Monovertex please confirm this fix. This was only a problem with people that had signatures in the upper levels like not the special members but the respected members.