Online auctions such as eBay may seem like the perfect marketplace for trading obscure collectors' items such as old baseball cards, postcards and Barbie dolls.

However, it is easy for rookie traders to stumble into common pitfalls such as splashing out on memorabilia that turns out to be worthless, or failing to attract any bids for their prized possessions.

Similarly, auctions and fairs can seem like a minefield for the uninitiated collector. So what's the secret to success?

Firstly, when buying anything on websites such as eBay, it is vital to check the seller's profile, particularly their feedback ratings and comments left by other buyers.

If you are considering bidding for items on eBay which could be fake, such as an autograph, it is important to check whether the seller has sold many similar items in the past. If they have, this indicates that they may be a professional dealer, which should provide greater reassurance. However, you should pay attention to sales within the last 30 days. If they claim to have sold an unreasonably high number of very rare items, such as first editions of Ulysses, within a short time, this may be an indication that something is amiss.

Many keen eBayers advise against buying anything from a seller who uses the "private" option, as this means you can't check what they have previously sold.

When selling online, the trick with websites such as eBay is ensuring that your item gets found, says John McElligot, managing director of eBay Ireland. The most effective way of doing this is to use terms in your description that match the terms that buyers most commonly search for.

At stamp fairs, novice collectors should only deal with traders who are members of the Irish Philatelic Traders' Association or with registered dealers. Many stamp dealers have a price list, but Eoin Boyle of IrishStamps.ie says these are not always written in stone. Be brave and negotiate, he advises.

Buying stamps in bulk tends to be better value than buying individually, so it can sometimes be worth purchasing more than you actually want and then selling off the excess stamps.

When trading rare coins at fairs, Veronica McCallian of the Central Bank advises collectors to visit a few dealers and compare what they have on offer before making a purchase.

Certain collectibles do not lend themselves to online trading, such as vintage cars, as these require a thorough inspection to ensure they are roadworthy. When buying an older car, you should bring a professional with you to assess it.