I do have simple USB scopes (commercial) that are simply a microcontroller with USB interface. Very inexpensive and very small.

Not something for looking at MHz digital signals, but DC to 100kHz bandwidth and up to 500kHz sampling rate. But a lot of fun stuff can be made with such a unit.

Next step up is something like a PicoScope. Real hardware to sample the data allowing 10Ms to maybe 1Gs with one, two or four channels.

Yes, used eBay scopes may have a huge number of features. But one interesting thing with some of the better USB scopes is that they may have a huge buffer compared to old traditional memory scopes. Having 1 million samples can be mighty nice when trying to find troubles with serial data.

Because of the buffer size, I often use a PicoScope instead of the traditional Tektronix.