If you're selling, make sure you at least get out of it what you got into it. But if it's Snapon, Craftsman, Wilton or "vintage", you can do a bit better than that. Don't be concerned if the item is broken or doesn't work, it's an easy fix, you just don't have the time. Remember to post very few and very blurry pictures, and don't give any brands or model numbers. Mystery builds value in the buyer's mind. You know what you have.
If you're buying, the ending price of 20% of the original asking price, will feel most comfortable for both parties. This is especially true if you haven't seen the item. The seller will feel reassured that you know what you want and what you're willing to pay, especially if you ask multiple questions to confirm what was stated in the ad.
Whether buying or selling, always make sure not to confuse the other party by conveying more than one piece of information in any correspondence. So for example, if you're asked where you can meet and when you're available, you should only respond with the word "Friday".
There are plenty more nuances to ensuring that both parties walk away happy, but these are the basics.
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