Know what you want in a guitar
Seems kind of obvious, doesn't it? But how clearly and narrowly you have defined your terms for what you are looking for in a preowned guitar most probably will influence how much you will be paying for it.
Let's base the assumption that you want a PRS Custom 22. That's a fine start and will lead to a number of preowned guitars that you should be able to find at any given time. You've either had your hands on one and played it or you've done your research and determined that it has the tone and features that you're looking for. You'll also find that the price will vary greatly from under a grand to several thousand dollars because, although you've got a specific guitar model in mind, you have not narrowed your options to limit this search.
Here is where you need to determine what you really want...condition? 10 top? birds? stoptail or tremolo? color? switch configuration? (fill in the blank). We can definitely find a value preowned guitar with this criteria providing that we limit our options. Why? Because we will be offered less choices by the more we specify or narrow our search criteria. Quite simply, fewer choices will mean a more narrowly returned search and means that we probably will pay in the upper range.
For example, let's say that currently on eBay there are 46 PRS Custom 22 guitars that will eventually sell from $900 to more than $3500...not bad at all. We also want 'birds', and we limit ourself by finding 25 guitars that match our criteria. '10 Top'? OK, now we've got 20 guitars from which to choose. Quilt? No problem, but we're now down to 8 guitars. But you've got to have a stoptail, don't you? Well, now you're down to 3 guitars that are going to sell for more than $2000. And even worse, none of them are in the cherry burst that you've got your heart set on so you're not going to find one this week at any price. Guess that means that either have to keep waiting or you've gotta pay full price and buy a new one, Jocko!
Now, waiting a bit is not bad and patience needs to be encouraged if you want to find the best buy in a used quality guitar...providing that your expectations and search terms are REASONABLE. But please remember, as pointed out in our example search, that the more choices that you have within your requirements for a guitar will result in a lower price and a better value in a guitar. So do yourself a favor and limit your primary ('know what you want') options when you're shopping for a used guitar and keep an open mind and consider all of the other secondary options.