Buying used when you should have bought new...

Too bad about your experience with the used transaction... I have found it really depends more on the manufacturer of the component, rather than the type of component. I've been lucky with my transactions in that the manufacturers have always stood behind the products:
Cables: I bought a pair of used Audience Au24 cables that were too short for my rig, I re-sold them w/o ever hooking them up to a member who found out that one of them was faulty, I offered him a refund, instead, he chose to send back to Audience and they repaired it free, no issue.
CDP: Resolution Audio offers a service where you give them the serial # of the used unit and they will reply to you with the component's history of its age, # of owners and repairs, they hapily service used units.
Speakers: Even though I bought my Green Mountain Audio speakers new, they offer warrantee for life on their products even to owners who buy used, really unbelievable.
Amps: Cary has been a pleasure to deal with. I bought my amps used and they were missing some chassis screws on the bottom and sides, called Cary and sent out the screws the next day, free. Later I had some upgrades done, they gave me a great deal despite the fact I wasn't the original owner.
There are a lot of great companies out there that offer tremendous service. Although, I'm sure I'll hit a product or manufacturer eventually that I'll have an issue with, so far I've been fortunate.

I always lean to buy new for the fair price.
Phono carts I only buy new.
Tube gear is simple to troubleshoot so I can buy it used.

...btw I would add motorcycle to buy-new list.

As mentioned above, used equipment should average about 50% or less of the retail price, while new equipment can often be found for 80-90% of retail. Therefore your savings from buying used should generally be about 40% off retail. One thing not mentioned yet is the added cost associated with buying used items you have not auditioned, then finding they do not work in your system. Sometimes you can break even when reselling them, but other times you will be down at least the shipping costs, unless you can afford to wait for the right buyer. If you like the cost savings from buying used, there are ways to reduce the risks, such as buying from reputable sellers (good feedback or dealer demos), buying more recently made equipment (which should not have significant wear or parts failure problems), and buying from still-active manufacturers who offer repair (and possibly upgrade) service on their equipment. Your example of using an item for a "few months" before it started acting up is unusual, and indicates the piece was probably working fine for the previous owner. Sounds like simply some bad luck on your part. I wouldn't give up on purchasing used, but you have to decide your own comfort level.

My first rule is to buy the best stuff, from the best places at the best price you can get. Then if you have a problem it won't be a problem to get it resolved. As the saying goes "The quality is remembered long after the price is forgotten." My second rule, if buying used the feedback rating is the most important thing. Yours or their integrity or feedback is everything. Buying from a good seller can introduce oneself to a much higher level of gear than you might be able to afford otherwise. I would tend to buy gear new that has wear items in it such as tape transports,cd players etc. They can often be found at the best discounts. Used from a high feedback seller for pre-amps,amps and speakers and such. And whatever you buy, obtain items that are not older than five years old. Time can affect anything for the worse.