Unfortunately, this is pretty rampant anywhere where you sell online. We have a couple measures to prevent it; your email is not shown in classifieds, and the contact form has a captcha. But, for the guy sitting in Nigeria that wants to spend the time to manually fill out a form and email you, the only protection is your common sense.
Also all members that join classifieds or forum are scanned through a database looking for emails or ips that match up with a spam database.
Here are some tips I list on the classifieds page.
DEAL LOCALLY WITH FOLKS YOU CAN MEET IN PERSON - follow this one rule and avoid 99% of scam attempts.
NEVER WIRE FUNDS VIA WESTERN UNION, MONEYGRAM or any other wire service - anyone who asks you to do so is likely a scammer.
FAKE CASHIER CHECKS & MONEY ORDERS ARE COMMON, and BANKS WILL CASH THEM AND THEN HOLD YOU RESPONSIBLE when the fake is discovered weeks later.
STEERPLANET IS NOT INVOLVED IN ANY TRANSACTION, and does not handle payments, guarantee transactions, provide escrow services, or offer "buyer protection" or "seller certification"
NEVER GIVE OUT FINANCIAL INFORMATION (bank account number, social security number, eBay/PayPal info, etc.)
AVOID DEALS INVOLVING SHIPPING OR ESCROW SERVICES and know that ONLY A SCAMMER WILL "GUARANTEE" YOUR TRANSACTION.