eBay is a risk. You might get something that works pretty well but you might not. You might have the skills to fix it, you might not. What if you need another scope to fix it?
Budget is the discriminator. How much do you want to spend? The next question: What do you want to see? 'Audio' only takes about a MHz of bandwidth so just about any scope can handle it. I wouldn't even think about a scope with less than 50 MHz of bandwidth on the off chance I might want to move on to microcontrollers later on.
The cheapest possible setup is a PC sound card. You'll have to do some Googling. I wouldn't use my best, or only, PC for this.
I bought a uses Tek 485 about 12 years ago on eBay for about $200. It still works well but I did buy a Rigol DS1054Z for the DSO capabilities. There are similar 485s on eBay from about $250 up.
I'm a huge fan of the Analog Discovery 2 but it's pricey compared to some low end used scope. The thing is, you wind up with a lab in a box. Dual channel scope, dual channel arbitrary waveform generator, 16 channel logic analyzer, digital IO with patterns. That's the simple stuff... Then there is the Network tool which produces Bode' plots of frequency/phase for various circuits. Or the FFT gadget. The dual channel power supply, although limited in voltage and current, is just right for powering breadboard projects.
By the time you buy a useful AD2 kit, it's getting kind of pricey. But it, and a laptop, easily fit in a backpack. You can work anywhere.
Philosophically, I like more channels. The 4 channels of the Rigol were more compelling than the DSO features. My 485 has only 2 channels and I wanted to be able to see all 4 signals of the SPI bus.
Probably the most compelling feature of the DSO is the single shot mode. You capture the trace and look at it as long as you want.
Today, I would be looking at the new Siglent offerings. I have my eye on a SDS 1204X-E for about $800. Four channels, 200 MHz and plenty of capability makes it a pretty nice scope.