Valuing coins is a complex topic as while the first stage, identifying the coin may be relatively simple, assessing the condition of the coin is something that takes years to learn. We've written a basic guide on the nature of coin grading here: Coin Grading Guide
A majority of valuable coins attribute their value primarily to their condition, rather than the coin type. For example, florins from George V can be had for as little as $15 per coin, but if sought in uncirculated condition, only 4 dates are readily available below $1000 and many are worth in excess of $10,000, especially if they are particularly high quality uncirculated coins.
Even identifying a coin can be difficult, some of the older proof coins for example are not easily distinguishable from the business strikes - the former usually being worth $30,000 upwards, the latter not often exceeding $2000.
Identifying your coins
The first stage in valuing your coin is to identify it - the Blue Sheet provides an identification page for Australian coins here: Value My Coin. Simply enter the year on your coin and it will show a list of coins produced for Australia that year. Then simply select the coin from the photo.
If your coin looks somewhat reflective, it may be a proof or specimen strike - the Blue Sheet's identification page does not list proof or specimen strikes as they are very rare and unlikely to be randomly found - you can however, manually navigate to the Blue Sheet page on the proof or specimen strike which will usually provide a more detailed means of differentiating the strike from a normal business strike. Note that coins may be artificially polished to look reflective - these are not what proof or specimen strikes are and such coins are frowned upon by the industry.
If you go ahead and click on the coin, it will provide some information about your coin along with a value of a typically circulated piece. If your coin is in better condition, it will be worth significantly more - I will explain more about this below in the grading section.
Grading your coins
The second stage is to grade it - I won't go into the intricacies of grading here - see my Coin Grading Guide for more information.
There are alternatives though, you can send your coin to a coin grading service - the two most industry recognized services are PCGS and NGC. Keep in mind that most other services grade to significantly different standards (usually weaker) and valuations provided by any mainstream reference would not be accurate.
The easiest way to get your coins graded by PCGS would be through a PCGS authorised dealer - you can submit coins yourself - but this may prove to be quite expensive unless you're sending over numerous coins. You can find a list of PCGS authorised dealers here: PCGS Authorised Dealers. The rough cost of sending a few coins yourself works out to about $50 per coin, if you're sending about 50 coins this ends up at about $30 per coin. Post-1955 coins will be about half and gold coins will be about double. Keep this in mind when dealers tell you their rates to get coins graded.
This process takes a few months but once your coins are graded you can enter the certificate numbers here: Certificate Lookup for an instant valuation.
In addition, now that your coins are certified, dealers can no longer argue about the grade - you can usually expect about 60-90% of the Blue Sheet value when selling but keep in mind that dealers will usually offer much less for lower value coins due to handling expenses.
If you don't certify your coins, you may be hit by the grade gap - some dealers may purchase a coin from you as EF, then go on to sell it as uncirculated or higher. This way they may seem to be offering you a high percentage of a catalogue value but in reality are offering you a small fraction of its true value.
When selling to dealers, you are best coming from a position of knowledge. If your coin is certified and you have the value - you know exactly how much to expect so if they offer too little, move on. In general though, unless a dealer needs that particular coin (e.g. he has a client in need of that coin), the price they offer won't be the best, but it will be the quickest - it's the only way to get instant settlement.
Another option, if you're not in a hurry for quick settlement is public auction. There are numerous public auction houses in Australia, all offering a different set of commissions. If you're intending to follow this path, research your options thoroughly, as commissions range from about 17% to 40%. Remember to factor in any buyer's commission that an auction house charges, as dealers will factor this in into their bids.
A majority of strong bidders at public auction are dealers, and while competition between them will prevent them from paying too little for your coin, you only get the wholesale price minus the commissions - so if you know the true value of your coin, you're better off going directly to dealers, saving yourself the commission.
The whole process of selling at auction takes about 3 to 4 months on average.
The other options are collector targeted auctions such as eBay or our Numis Bid. The advantage of these is that most of the bidders are collectors so you can expect close to retail prices, commissions are usually very low (rarely exceeding 10%). The disadvantage is that they don't usually offer a viewing option for bidders - this means that coins that haven't been certified tend to realise lower prices, factoring in the uncertainty of condition.
In addition, if you sell on eBay you will be required to photograph and describe the coin yourself - most new sellers tend to do this very poorly on this which results in very low realisations for their coins (sometimes this makes sense though, why spend $2000 on a camera for a $200 coin?) If you choose this option, make sure to provide clear photos, if you can't provide a third party grading, your bidders will need to grade the coin from the photo and they will need a clear photo (a very clear one). The whole process including auction, payment and postage takes about 2 to 3 weeks and if done well can realise close to retail prices (minus about 6-10% commission).
Keep in mind factors such as receiving payment, posting and insurance, marketing of your listings, etc - we recommend reading one of the numerous books available on selling via eBay before listing your coins.
Numis Bid on the other hand will describe, photograph and estimate the value of your coins before listing them, similar to a public auction, except with a collector bidder base. You can learn more about selling on Numis Bid here: Selling on Numis Bid.
The best option for selling your coins depends primarily on how quickly you need the money, the type of coin and whether it has been certified. Remember to always research your coin thoroughly before looking to sell it.