Re the DSO138 kit scope, when last I heard, Banggood sold authentic JYETech stock. Actually the one to get is the DSO150, which is a newer design with a rotary encoder and slightly better specs. Looks like the DSO150 kit version is about $16, plus you need to buy a real scope probe (comes with alligator clip probe) and a 9V power supply. Assembled, with a real probe, but still no power supply, is about $23. The fun part is converting them to battery powered, so another maybe $12 for a charger module, boost converter module and the battery. There's a long thread on the JYETech forum about this battery conversion, with everybody, including me, giving their version of how to do it.
It has been surprising to me how useful the DSO150 has been. You would think it's too slow to be of any real use, but there are a lot of things like steppers, servos, PWM, various serial protocols, audio, and so forth, that these little scopes deal with just fine. And it's also useful to to have a battery-powered scope that isn't referenced to anything. So if you want to measure the voltage drop across a mosfet, you don't have to worry about where ground is, or blowing anything up. But probably because of the small capture memory, I believe triggering only works at 20ms and faster.
In the video, I think Dave has the 138 on Auto mode, and I think it wouldn't wander if it was in Normal mode.
And for the adventurous, there's a two-channel version (the Wave 2) for $65 plus a battery. Does X/Y, has a function generator, etc., but basically the same specs.