A car is listed with "Seller: SIXT Titling Trust" — what does that tell me about its history?

This indicates the vehicle came from a rental fleet, since SIXT Titling Trust is the legal title-holding entity used by the SIXT rental car company in the U.S. Former rental vehicles are typically well-maintained on a fixed service schedule but also see higher mileage and more varied driver wear than a personally owned car. It's a fleet origin marker, similar to seeing Avis Budget Group or Hertz listed as the seller on other vehicles.