Ebay is an incredible place to find new and used electronics. Gold detectors specifically, metal detectors tuned to the frequencies for finding gold are available from many quality makers and regularly cycle through ebay. The brand of detector you buy and the model depends on the specifics of what your target is. Two catagories exist in detectors: VLF or Very Low Frequency and PI or Pulse Induction, both types can effectively prospect gold nuggets. VLF detectors descriminate much more effectively the non-target metals, so that less general digging is necessary. PI detectors on the other hand have some advantages over VLF detectors under certain circumstances. PI detectors have increased depth of detection and they are not disturbed by severely mineralized ground. VLF detectors that are designed for Gold nugget detecting have frequency ranges between 6-71 Kilohertz, most are single frequency but a few use multiple frequencies to cater to varying conditions. A high frequency machine like the Fisher Gold Bug 2 at 71 Khz. allows this VLF detector to be very sensitive to tiny gold pieces. A lower frequency machine like the Whites MXT at 14 Khz. will enable potentially greater depth and have substantial power to detect larger nuggets, but not as sesitive to find tiny gold.
PI detectors operate by shooting pulses of energy at time frequencies, the more times per second the more sensitive the machine. Minelab has several model of PI detectors which are used almost exclusively for nugget hunting. The 2100, 2200 and 3500 represent a powerful lineup of gold PI detectors thought by many to be some of the deepest detecting units. The detecting coil varieties for the Australian made Minelabs is truly diverse and the extra large size of these coils is part of the reason for their incredible depth. Large coils are being tested on the Garrett Infinium, a powerful land and water PI detector as well as with the VLF model the MXT. These detectors with 18" coils may come very close to the Minelabs depth and offer water mobility and/or greater discrimination.
With many pieces of equipment the old adage "you get what you pay for" is true, this is especially true with quality gold detectors. Even used, expect to pay at least $250 for a quality gold detector. The detector you buy depends on several factors, the most important being your budget.
The features of the detector are important, for example a manual balanced machine VS. automatic ground balancing. A first time novice should really consider getting an automatic machine because manual balancing takes time and skill to learn and varies from machine to machine (a great source of frustration for beginning detectorists). Technology has changed dramatically over the past ten years and detectors are getting better. Many older models do not compare to the better modern gold detectors.
This article is not intended to push any one brand over another, the truth is that the very best detector in the hands of someone who doesn't know the machine will produce far less gold than an experienced prospector with confidence and only a fair detector on his/her side. All detectors take many hours to master, some machines, however, have a much faster learning curve.
Using price range as starting point. Some of the better and more popular gold detectors in the $200-400 range include the original Lobo, the Goldmaster 3 and 4b, the xt17000 and the original Goldbug to name a few. From $400-600 a great machine can be purchased and some of these include the Goldbug 2, the Lobo ST, the MXT and GMT,  the xt18000 and Eureka Gold. All of the previous machines are quality VLF detectors designed for land use. Always look for the "extra's" you get with the detector, I recently purchased an MXT that came with an extra DD coil, headphones and coil protectors. These were nice bonus' that would have cost ~$200 to buy.
Any PI detector can be used to try to detect Gold Nuggets, some are designed to be more effective at this task. Gold is a very difficult metal to detect and thus a machine that can pick up gold will really cry out when other metals get under the coil. Trash is the biggest burden of PI machines because many models do not have different tones or discrimination settings to determine a tin can from a nice nugget. The Garrett Infinium has an interesting high-low, low-high pitch system that can give some discrimination. This machine is versatile being able to go underwater, and packs a sticker price far below the minelab PI machines. Minelabs do have some descrimination as well, however, no PI machine will descriminate like a VLF detector. If depth for finding large nuggets is what you want and don't care that you may have passed 60% of the gold bye because it was all small pieces, than a PI detector is your choice. In a nutshell, a VLF detector will find many more gold pieces in a day because most gold that is left in the ground is quite small. If you are in severely mineralized soil or are working tailing piles that go very deep and want to scan 3 feet down then a PI machine is the right choice. No one machine does everything and most hard-core electronic prospectors use different detectors for the different locations and conditions that exist. If the gold is abundant but is in very small pieces then a PI machine would not be very effective. If the ground is full of "hot-rocks" and "cold" patches that throw your VLF into cry-baby mode then it may be better worked by a PI machine.

Judah Sanders is co-owner of Dragon Pearl Tea company and an avid amatuer gold detectorist. Favorite place to detect is old hydraulic mine tailings and tailing piles near productive river areas.