There's not much to a speaker. I should say that compared to an av receiver there's not much that goes into a speaker. You have the drivers, tweeters and woofers, a cabinet, circuit boards, some insulation, a grill. That's simplified and hardly scientific but I'm trying to get a point across.
Your speakers usually last much longer than other audio components. There's just not much that can go wrong with them. That is unless you overdrive them and blow out the drivers themselves. You can tell if a driver is blown by just listening to it. Or, if it's a woofer, carefully push the cone in with your hand. Do you feel or hear a grinding or crunching. If so the speaker is blown. You'll need to replace it.
In fact, it's quite easy to replace parts of a speaker that goes bad. The most common ailment is that the "surround" which attaches the speaker cone to the basket becomes deteriorated. Here's a diagram.
You'll first notice a bad surround as the speaker will begin to buzz or make a fart sound when there's low bass tones. On inspecting it you might find that there's actual holes where the surround has decayed. Simply touching the surround with your finger may cause it to disintegrate as you touch it.
I've got a pair of McIntosh XR6s that had bad surrounds in the largest of the woofers. Here's a couple of pictures.
In the old days surrounds were made of cloth, and though the sound may not have been as good they didn't go bad.
I removed the woofers from the cabinet and took them in for replacement of the surrounds. It's much better than trying to transport huge speaker cabinets.
There's a few places in Cleveland that repair drivers. I took mine to Audio Craft. Another downtown shop is Empirical Sound. I believe that Elctra-Sound on Brookpark Rd. also offers this service. It's best to call a store first to see if there's a surround available for that sized driver. You could do it yourself but it's a bit difficult and the glue used is somewhat toxic meaning a ventilating mask is needed.
Once the surrounds are repaired the woofer is as good as new and that's where pricing on used speakers becomes tricky. If you go by Bluebook, yes, just like cars AV gear has a Bluebook, the price on used speakers aren't reflective of any sort of repair work you may have had done. Speakers can last a long time as long as you take care of the cabinet and don't crank them too loudly. It's just that in the used market you won't be able to sell them for too much. On the other hand, you can find some great deals on used speakers.
If you have a question or if I may be of service email me at firstname.lastname@example.org