IN ORDER TO AVOID COUNTERFEIT CHINA, HONG KONG OR SINGAPORE COINS & STAMPS FROM CERTAIN UNSCRUPULOUS DEALERS/SELLERS FROM THESE LOCATIONS, DO THE FOLLOWING: 

• Deal with established USA, Canadian, British or EU dealers, many of whom you can find on eBay by looking at their "About Me" descriptions or auction descriptions.  They will generally list if they deal in China, Hong Kong or Singapore stamps and/or coins plus some will list their current auctions of them or have list available in eBay store.  There may also be a link in "About Me" to their web site where more information is available. 
• What's more most eBay listed dealers want PayPal payments and many accept credit cards so that winning bidder/buyer has recourse in case any fakes slip through.
• Also, all legitimate dealers will accept return of item if you have evidence that it is fake.
• As soon as stamp or coin is received, if you do not have sufficient expertise to determine if it is genuine, you should look up philatelic or numismatic dealers in your area with expertise in your Asian stamp or coin purchase.  They will generally be pleased to let you know if it is genuine or not, in hopes of establishing good relationship with you for future business. 
• If item is valuable enough, you should consider paying for a certified appraisal for insurance purposes particularly if you won eBay auction since it may be worth substantially more than you paid.  If so, you may want to obtain replacement value loss insurance in case of fire, theft, natural disaster, etc.
• Store your stamp or coin in accordance with philatellic or numismatic accepted practices.  For stamps, they should be placed in an album and either enclosed in a clear plastic pouch or less preferably, hung by a stamp collection hinge.  For coins, they should either be placed in a felt lined case or placed in individual clear plastic coin cases.  Depending upon value, you may opt to keep your stamp and/or coin collection in a vault, safe or other secure storage facility.
• Since stamps and coins have both artistic and intrinsic beauty, feel free to take them out periodically to enjoy their beauty; however, always handle them in accordance with philatelic and numismatic standards, i.e.  If unsure what these standards are look them up through Google search or ask your dealer.