In order to buy collectibles from your friend/neighbour, both of you have to be at least level 9 and have the City Market unlocked and built. Once these requirements are fulfilled, you can visit your friend, tap on the City Market and see the collectibles/materials that he/she had put for sale. Please keep in mind that you won’t be able to see items/materials which you haven’t unlocked yet (items which require higher levels than your current one) in your friend’s City Market.