<SELL itemid="[color=red]<ITEMID>[/color]"/>
I'm surprised how the items even showed up in 'Buy' section of the shop since they're cash items. Weird :O.
Don't remove the iscashitem attribute, change it to 'false'. Then, rename all item id's using Aristrum's zitem tools and generate a shop.xml from the same. If you're using the 2007 files, search for all constraints in Olly and type 500000 and change it to whatever you want.
If you're using 2008 files, I believe gregon posted the edits.