Check if it's legal: Different countries have different rules. For instance, many countries do not allow posting ads of animals it considers culturally, ecologically or religiously significant. Check whether the country you're in permits selling whatever you want to sell.
Read and follow the Ad Posting guidelines: Most online classified sites will have a set of Ad Posting guidelines. Don't just click "I agree"; read the guidelines in detail.
Category: Be category-wise : Choose the correct category: If you are trying to rent out an apartment, but by a small mistake, you posted under Villa, your ad may reach the wrong eyeballs.
Images: Post the best images: A picture is worth a thousand words. Posting a picture of whatever you're trying to sell helps people immediately understand and relate with the product. Choose the best pictures (but don't use fake pictures!).
Language: Stay simple: Use easy-to-understand language. Describe the product in simple language, but don't make it too lengthy. Add a list of documents - e.g. mention service record, if you are trying to sell a car.
Research: Buyers are not fools: Buyers check at multiple places before they finalize sell. So keep your pricing realistic, neither too high and nor too low. Also, don't post prices like USD 4,444 or 1,111. They are almost always fake.
Never post duplicate ads: Often new users post multiple ads of the same product, thinking multiple ads will give them faster or better response. That's an incorrect practice. Special software will immediately trace and remove that ad. So post only one ad.