Item Types "Sellable" and "Signup" are used for items sold at a set price - usually to multiple buyers.
Choosing type Sellable or Signup changes the item form to display additional purchase options (and replaces bid fields with a price field):
The Signup type is best for "pay to play" parties and activities because a built-in report provides a list of buyers for the party/activity host (with the names and contact information).
Online Signup items also provide the option to display the buyers' names (or online handles) in the purchase history of the web catalog so prospective guests can see who they'll be joining at the event. The name display can be specified at Site Settings > Items > Catalog > Additional Online/Mobile Settings.
A common approach to the tricky question of what to list as Fair Market Value (FMV) on items where someone is hosting a party, outing, activity and/or classroom projects is to base the FMV on the cost of food/materials/venue costs etc. This will provide documentation so the item donors can claim a charitable contribution for their out of pocket expenses, and buyers can claim the difference between what they paid and the FMV.