Question 1: 5G is not worth it today. That answer will change in a few short years. I can definitely see the need for high speed, low latency, low overhead protocols. As it is, 5G is being deployed on new frequencies only, 2600mhz (2.6ghz, aka, a little above old 2.4ghz wifi). It will need to be deployed on the lower frequencies like 650, 700, 850mhz to get the coverage(wall penetration) that everyone is used to. That won't happen till people let the carriers retire 2G/3G and deprioritize LTE. Its a legacy issue like switching to AC power, or away from Imperial measurements. End users need to spend money and accept change. The big benefit to 5g is the multiplexing (think increasing max occupancy limit of a room without any physical changes).
mmWave won't be good as its in the super high range of 60ghz-100ghz. At that frequency, you'd have to be putting more power into broadcasting. The current 'signal' limitations of 3G/LTE was never the cellphone tower's broadcasting strength but rather your cellphone's broadcasting strength. More strength needed(higher mhz = higher energy), more power consumption. There is the trade off where if you can do the job faster, you can rest faster (mmWave), but thats even more a fringe case where 5G is about optimization. mmWave is only really an option cause the spectrum is cheap and plentiful.
2. Yes, they can be trusted if you know what to look for. If you don't, well, its really no different from walking into a used bike dealership and asking the salesperson with the nicest smile.
- Check if the phone was stolen. Check the Status of Your Device in Canada - DeviceCheck.ca Any seller who won't let you see what the IMEI is is not worth your time. Always try to check the IMEI from the phone software. Dial *#06# even if there is no sim card in the phone.
- Kijiji phone sales in empty parking lots is one of the most common setups for a mugging.
- Google everything. You can find model numbers and information on gsmarena.com including international models.
- Assume no warranty, no refund, no exchange.
- Dual sim is not a selling point. Its a feature requirement for those that require the feature.
Now you know why most people just suck it up and buy from a carrier or big box store.