To answer some questions and clarify a few points:
-"Dual channel" means you can hook up two probes and track two signals at once. The principle application of this is to use an enabling signal, such as /CS (chip select) on a RAM chip, to act as the trigger so you can see the relationship of the data and address lines (among others) to this central signal.
-"Dual beam" means you have two guns painting two traces on the display at the same time. This is prefered for two-signal work. Most lesser two channel 'scopes have one gun, so either display one channel, then the other; or do what's called "chopping". Chopping is where you show one signal's trace for a small period of the total sweep, switch to the other to show a small portion of that, and keep doing this switching throughout the sweep period of one full display. The problem with this is you run the chance of missing some key event because something ends up getting cut out because the chop is looking at the other signal at the time.
This is where modern digital scopes with LCDs shine. Both signal are read and digitized, then both are displayed on the LCD at the same time.
-"Digitizing" simply means the incoming signals are measured on a timed basis, as binary values, then stored in onboard RAM for later (near instant) display. The better scopes allow you to take one reading only (sort of a "freeze frame"), then study it at your leisure or download it to a computer for later, deeper analysis.
The vast majority of the scopes you find on Ebay are servicable analog scopes. These typically have two inputs, 20MHz to 100MHz, but one gun, so supply chopping controls, with all the limitations mentioned above. The early DPOs (Digital Phosphor Oscilloscopes) will give you "freeze frame" ability, but the storage is you and a camera, as the results only stay on screen for a short time. And they aren't cheap, easily more than the $150 you mentioned.
There are pros and cons to analog and digital scopes:
Analog will still do 98.5% of what you want to do in electronics work, IF you are only interested in frequencies, levels and rise/fall times. Which is what most electronic troubleshooting is about. It excels at showing minor discrepancies in a continuous signal because you see the irregular, infrequent events as dimmer signals amongst the strong, regular signals. You can catch ringing and overshoot problems easier, for instance. But they suck at catching one-shot events in the middle of a lot of traffic, or in long periods of no activity at all (seems like?). But, a modern digital scope with extended triggering capabilities will catch these rascals easily.
If you must catch infrequent events reliably, with expanded triggering capabilities, or store traces and download them to a PC, then digital is the only way to go. Even units running less then $400 will do this. I saw an ad in an email recently (search these forums, or Google, using "OWON", as I believe this has been covered here before) of a two channel scope that has 25MHz bandwidth with 100MS/s (megasamples/second, the more the better) with must-have features comparable to my $2400 Tektronix 4-channel color LCD scope. Of course the manual is Chinlish in the worst way (but usable) and isn't very informative about using the USB port, but it can download displays to a PC for printing. It has a color LCD; variable phosphor (nice!); and a limited, but usable, built-in math package. It has a measurement bar on the screen's right that allows you to track five values on-the-fly. One ad even says they throw in a free battery, which suggests portability, but I couldn't find anything about installing the battery in the manual. Beware!
Amazingly, it has more memory per channel than my Tek!
I hope this answered more then it confused.
kenjj