Legacy components - less reliable?
Let's get the obvious matter out of the way first: isn't old equipment supposed to be unreliable? CD transports have mechanical parts that wear out and the lens doesn't last forever and amplifiers have capacitors that get less with age. Indeed, this cannot be denied. But it depend greatly on the quality of the used parts and also on how the previous owner treated its equipment. I have been buying huge numbers of components on the used market for over 15 years and have experienced only a couple of problematic components. Especially amplifiers have proved to be very reliable. Never have I come across worn out capacitors. I have experienced other problems such as software errors, or worn out relais, but these are usually things that can be fixed. I have also had cd players that skipped or had other silly behaviour, but even here, the number of drop-outs couldn't have been more than one out of 20. So while I cannot deny that there is a risk, you're also paying half the newprice and it's a risk worth taking.
Upsampling and the ongoing progress of all matters digital
Even though the recently (I'm talking around 2008) upsampling became all the rage and it seemed that old fashioned standard conversion was doomed, lately we see another trend: going back to basics. Several big name brands are unveiling players that use the ancient Philips TDA1541 multibit converters, there's Harmonix that uses very old JVC conversion and there are companies that boast non-oversampling and even non-filtering. What this means is that things are a bit relative and subject to hype. Of course I will not deny that advances and even improvements in digital have been made, but it doesn't neccesarily make for better sound. Especially is you buy a previous reference, you're unlikely to easily better it by buying new.
State of the art - back then
There's a school of thought that says development goes so quick that 5 year old techniques are antiquated today. They may have a few arguments here when it concerns digital but when it comes to amplification, good old linear powersupplies and bipolar transistors are still not obsolete. In fact, many manufacturers still use techniques invented decades ago. So again, if a product was state of the art once, it may not still be state of he art now, but you bet it still stands strong. And don't forget that you're paying half the newprice. If you take that into consideration, a new amp may have a very hard time to match the older reference at the same price. I haven't come across amplifiers that succeeded yet.
New "digital" switching amplifiers
These new and exiting products can indeed sound very good. They are very energy-efficient. And they consume almost no power when idle. But I haven't come across a switching amp that I like better than the Rowlands from the nineties that I have been using for a long time. If you're only looking for detail and transparency, and don't mind a slghtly moe artificial sound, then switching amps may well be the choice for you. Heck, you can even buy them cheap these days. But they're not quite the equal of solidly engineered "old fashioned" analog designs.
BOTTOM LINE
While I cannot deny that there is a risk, you're also paying half the newprice and it's a risk worth taking. Speaking from experience, in the 15+ years that I have been shopping on the used market, the percentage of drop-outs really didn't exeed 5%. And this includes digital products. However, be certain to have everything packed really well, because couriers can and will drop a box every now and then...