Separate bullion from numismatic value
The first question for any coin is whether it is worth more than its metal. Modern gold and silver bullion coins, worn sovereigns and common silver trade close to spot price and move with the metal market; they need no specialist treatment and sell quickly. Numismatic coins — those valued for rarity, date, mint, striking or condition — are a different market with different buyers.
Mixing the two in one lot is a common and costly error: a genuinely rare coin buried in a bag of scrap silver is bought at scrap weight by whoever spots it first.
Understand grading before you form expectations
Grade is the dominant price factor for most coins. The Sheldon scale runs from Poor (P-1) to Perfect Uncirculated (MS-70), and the price curve is steep at the top: the step from MS-63 to MS-65 can double or triple a coin's value, while the step from Fine to Very Fine on the same coin may add a modest percentage.
Strike quality, lustre, toning and eye appeal sit alongside the technical grade. A technically lower-grade coin with original lustre and attractive toning often beats a higher-grade coin that has been dipped flat.
Never clean a coin
This is the most important sentence in this guide. Cleaning — polishing, dipping, scrubbing, using metal cleaner or even a vigorous cloth — leaves hairlines and strips original surfaces. Experienced buyers detect it instantly and discount heavily, and 'cleaned' is a permanent description that follows the coin.
Toning that looks like dirt is frequently the original surface acquired over a century, and it is desirable. If a coin is genuinely soiled, leave it and say so; let the buyer decide.
When to slab and when not to bother
Third-party grading (PCGS, NGC and their peers) encapsulates a coin with an assigned grade and an authenticity guarantee. It reliably increases the price where the coin is high value, where the grade is borderline between two big price tiers, or where the series is heavily counterfeited.
For coins worth less than roughly the cost of grading and shipping multiplied by three or four, it is a net loss. Raw coins with good photographs sell perfectly well in that band.
Lotting and cataloguing
Significant coins are catalogued individually, with the date, mint mark, denomination, reference number, weight and grade stated, and photographed both sides under even light. Lesser material is grouped by country, period or type, which suits the dealers and collectors who buy in volume.
Supply everything you know about where the coins came from. A collection with a documented purchase history from a named dealer or a dated inventory list is easier to sell than an anonymous box, particularly for issues where forgeries circulate.
Costs and alternatives
Auction takes a seller commission and the buyer pays a separate fee on top of the hammer price. Against that, competitive bidding is what finds the true price for a scarce coin — a dealer's outright offer will be below retail by design, because the dealer must resell it.
The practical rule: bullion and common material can reasonably be sold outright for speed; anything with genuine rarity or grade upside benefits from competition.