They are free to use here.

1) Take high quality pictures and lots of them of each item. If you can't get pictures and upload them get somebody that can. Nothing irritates me more than no pictures or poor quality pictures of something. No picture I skip right on by.
The first impression of your item is very important as to whether somebody calls or emails about it. 

2) Have some sort of contact info either phone or email.

3) Post a good description, noting items of interest like age, hours, model year, condition, regular maintenance, major repairs, or other known issues. Your not being charged by the word like in conventional advertising so be descriptive. More info is better not less.

4) Be aware of scammers, buying for others especially those claiming they are at sea in the service buying for family members, wanting to overpay and have you refund the difference. Also be aware of unscrupulous types that claim they have willing buyers for your items. If they want they can buy and resell on their own don't allow them to set you up with a buyer as they charge outrageous fees for supposedly putting you in contact with willing buyers and may not even do that. 

5) TBV (trust but verify) everything and everyone in regards to a transaction. Be wary of payment scams and methods. Make sure funds clear before releasing items especially those with titles.

Be alert be aware and be honest. Lots of great folks and great success stories on the classifieds but your on your own if you have issues.