Unfortunately, with the exception of the rule "Cars should be built by the Cub Scouts" (which can be difficult to prove at best), there is nothing in the rule sheet included in each derby kit that would disqualify an engineered car bought on eBay. This is why units should always distribute their own official rules well in advance of the race.
I would recommend any unit include the following in their Pinewood Derby rules:
1. Tigers/Cubs/Webelos shall build a new car and shall not re-race a car already entered in a previous year's Pinewood Derby.
2. Tigers/Cubs/Webelos shall not
