Hearing aid auctions include a required FDA disclaimer. Read it. But here is the jest of it.
Hearing loss is complex and most people should take any hearing aid (HA for short) to a specialist to get the most from it.

Naturally any used HA was specifically designed for another person. Your needs will be different and that means a hearing test. PLUS, digital HA have to be programmed by special computers in order to fit them to your needs. In fact, a new HA needs to be tailored to you as well.
Your savings may not be as good as you expected after you add in these costs. A hearing test, fitting the ear piece and programming can easily exceed a few hundred bucks. And remember that the guy you go to won't make a dime on the HA, so he has to make a living too.

What this does not do is recommend any manufacturer or model. The science behind HA is too complex and too dynamic for that. What's good today will be outdated too soon. This does give you enough info to understand the good, the not so good and how to keep from getting scammed.

This is meant for ebay folks but understand that there can be a tremendous markup on HA. There is a wide range of prices and specialists know how to disguise true costs. I cannot begin to cover this part of buying HA.

Now for the deep stuff.
When I look at hearing aid auctions, I see folks are often bidding too high. The purpose of this guide is to help you understand a very complex device so you can better decide which HA is best for you and to get a good deal. A really bad scene is if you buy a HA that is a big disappointment.

Even though HA are complicated, it is not too hard to learn what to look for.

To begin, it helps to understand just how complex our ears are. If you are hard of hearing, you have come to realize that background noise (radio, TV, noisy crowds) makes it harder to hear than when your hearing was normal. With normal hearing, we, filter the background noise and also help our brain focus on the sounds we choose to hear.

Scientists are beginning to understand this and are engineering HA to make them approximate normal hearing. The better they do this, the happier you'll be with your new HA. Inexpensive digital HA amplify all sounds, even the background noise. You hear everything better but the background noise is very annoying.

The way HA amplifiers are improved, are with algorithms.  (A HA is just a tiny sound amplifier - like a HI FI but much more complex.) These algorithms are complex mathematical formulas that shape the sound frequencies. But you don't need to understand them, how they work or even their fancy name. Thank goodness!!

But it is important to know they exist and that they ONLY are possible with DIGITAL HA. (Old fashioned analog HA are out.) In fact, this is a new and expanding technology. This means that the newer HA have more features. And the more features, the better the HA.

I wish that all you needed to know is that a digital HA of any kind will solve your problem. NOT SO. Digital HA range in price from a few hundred bucks to over 5 grand (retail). It doesn't take a rocket scientist to see that there must be a BIG difference.

Now comes the hard part. How can we determine the good, the bad and the ugly?
1- DO NOT BID (ever) on a HA when there is no MODEL given in the description (the more info, the better). All manufacturers make cheapo HA. Ask the seller for more info.
2- We can't rely on price (sellers may bend the truth), or they may have paid too much for their new HA.
3- Dates are fairly reliable and are helpful because - the newer, the better. A 2005 to 2007 HA may have better features, when it is one of the more expensive HA. HA made before 2000 have limited technology.
4- Do research. Copy the name and model into your search engine. Be willing to spend at least half an hour on this - or until you find out what you need.
5- What is your hearing loss? There are four grades of HA amplification: mild, moderate, severe and profound. You need to know your level as too weak does not amplify enough and too strong will not work at all for you.
6- See below for additional advice.

A final kicker. The more hearing loss, the more important are the complex (translate as expensive) features. The main reason is that the greater the HA amplification, the more trouble you have with background noise (unless it is suppressed). Also, if you’ve a severe loss in certain frequencies, you may need more "channels" to shape the sounds.
Channels are like a stereo equalizer. Each channel is really a mini amplifier. Expensive HA will process channel frequencies not only to amplify but to process sounds using algorithms (that dirty word again). Using channels is how they minimize background noise. But let's leave the geek stuff to others.

Hearing specialists have programing equipment that modify the channels to fit your needs. Four to six channels should be sufficient as most HA have a frequency range of about 50 to 5000 HZ. Our normal hearing ranges from about 25 to 16,000 Hz. You see that hearing loss in very low and very high frequencies are not amplified.

What should you look for?
The biggies:
a) Must be 100% digital
b) Noise cancellation, reduction;
c) All features that mention automatic operation;
d) Feedback elimination;
e) I like open ear HA. They give a more natural feeling.
f) The new thin tube (BTE style only) offer several advantages. Available since 2007.
g) Make and model so you can research the HA
IMPORTANT: If a feature is NOT mentioned, it is not included. I don't care how good the review sounds.

Of lesser importance:
a) Two microphones indicate a better quality HA
b) Number of channels, (minimum four to six);
c) Telephone mode or program
d) A class D amp gives long battery life.
e) Number of programs/memories (two will do but the more expensive do more and do it automatically by using channels).
f) Look for a connection port (used to program digital HA)

General Info:
There are four HA styles: CIC (completely in the canal) 2; ITC (in the canal); ITE (in the ear); and BTE (behind the ear). I mention this because of the varying difficulty in making the HA fit your ear. The least expensive to modify is the BTE. Unless you are lucky, the others may need professional help. More people prefer CIC or ITC styles so the better deals are with BTE.

Once you have your HA, you can go to a COSTCO hearing center for low cost services.

Good luck. There are some good buys but you need to understand the product first. This url has some good info - www.ahearingaid.com (includes price and feature comparisons on recent HA)