Obviously buying older used gear can be a good way to squeeze more value out of your budget.
Yes, there are risks that go along with buying used, but many people do it so as to be able to get the most out of their audio $$$s.
The key is to ask the seller the right questions up front to help decide if a) the seller is trustworthy and b) the item is worth what you are paying.
Its usually a good idea to ask things like why the seller is selling, what is the ownership history of unit, are there any known defects or functional or sound quality issues.
Also know what something used is actually worth these days. DO NOT OVERPAY!!! That way if things do not work out, you can resell and move on without taking a big financial hit. Be aware of shipping costs as well in taht these can become large for larger heavier items that are not local. Buying local only is usually a good idea to keep costs low and reduce risks. You might even be able to audition in advance of buying.
Following sales on ebay is a good way to get an idea of what something is actually selling for. Audiogon blue book (pay by the month) is a good investment to get an idea what more popular high end audio items are selling for.
Stick with more common and popular products that have multiple professional reviews and many individual reviews available for them in order to help minimize risks.
Avoid one off or boutique type items that have little information available about them in general. The internet and all the info on it is your friend when it comes time to make buying decisions!
Doing the homework up front is always key to making a good buying decision, especially when the financial stakes can be high.