How to start: community or the product?

Hi guys! I actually new in startups, but I think I can really be useful for this world and I have an idea! I will grateful for any advice or critics for my plan

The problem

National or international parcel services are too expensive

The solution

For people who travel and have extra space to take some stuff, we can collaborate with travellers and parcel requesters to save some money for 1st ones and reduce shipping spends for 2nd.

Requirements to make the business work

• Robust contracts between travellers and requesters to protect
  • travellers from forbidden stuff in parcels
  • requesters from stealing/damaging parcels
  • both from compensation misunderstanding
• Simple and clear finances transfer and guarantee
• Insurance possibility

What I need to start

• A community, as huge so finding the right traveller won't be a problem
• Software:
  • to seek and post travels
  • provide transactions
  • verify users

How to solve it

• To maintain 1 direction (e.g. Moscow-Antalya) I will be enough to have someone who travels every week there and back, so maybe it will better to start from one and then expand to others.

Qs to Answer

• Where to start? It's okay to create a group in FB and promote it across the travellers and expat groups
• Have I build an app itself immediately to suggest users a productions-ready product or try to build a community than start the development?