Luno started as just a centralized exchanges were you just send your bitcoin to them, then they send you your local currency in equivalent to the bitcoin value you sent. Not until further upgrade, luno added the direct exchange option where you can trade directly with other customers through their Peer2peer option using the platform as escrow. Localbitcoin on the other hand has always been a peer2peer exchange meaning, Mr Trump advertise to sell his bitcoin at a particular price, Mr KIM agrees to buy the bitcoin then the platform serve as an escrow and that's all.
Transaction done using Local bitcoin are not reversible that's why we have high rate of scam report associated with the exchange meanwhile hardly before you hear a report of users getting scammed on Luno mostly because of their high level of security verification clearance needed to use the App. And yes Luno buys most of the sells order at the lowest price possible and profit by sending it to other customers. Notice that when i tried exchanging directly from my wallet without visiting the exchange platform.