Yes, as long as you have a purchase receipt and or register it in your account (these days, I believe the devices register automatically as soon as you sign into your Samsung account on them.), the full manufacturer's warranty will be in effect. I do not believe you can purchase Samsung Care through those vendors, but you can buy a protection plan from Amazon at time of sale. You would get a bill of sale from Amazon. Why would you buy something from a vendor who won't give you a receipt? In the case of Amazon, your order is your invoice, however, the date of sale becomes the date the warranty starts (not the day you receive it). I don't know about swappa, but you would probably still get a receipt/order confirmation from whoever you purchased it from.
If you buy a refurbished phone, there's no guarantee there will be a warranty. If there is, it would probably be 90 days. You would have to check that information prior to buying.
