What kind of marketplace are you looking to create?
Personally I prefer sites that are unique. Buying a script will mean that your site as well as others are basically clones of each other with very little between them.
A custom made site, although more costly, offers more flexibility and also gives a site its uniqueness that pits it above from the rest.
I too like uniqueness in things but I can assure you that the majority of buyers prefer to get a good price over a good looking store.
If he can do both then he will make a killing.