Look, I myself approach sellers about trades and sales that don't eventually come through. Everybody does. The question is how you turn people down. So instead of sitting here enjoying my heartburn and expecting a heart attack at my tender age, I decided to share my view on how to treat sellers and approaches with courtesy even when you are not interested in further communication.
- Do unto others as you would have others do unto you
This is indeed the golden rule. Calm down, and think how you would like to be treated if you were the sender of the undesired offer, then proceed to step 2.
- Always write a short and polite reply
Never leave a message unanswered, it can really hurt the sender. Got a trade offer from a shop you are not interested in? Send a nice reply saying "no thank you". Got a convo telling you you are in a treasury? Thank the sender. Got an offer you are not interested in? Reply and say "thank you, but I am unable to accept at this time". Remember, nobody can read your mind or guess you taste, even with the best intentions. So be nice to people who took the time to tell you they appreciate you. You don't have to get into debates, or promise things you don't mean. Just..
- Say what you mean (and don't say it mean)
Never leave people hanging. If you asked for an offer for a custom listing, and didn't like what the seller suggested, just tell them so politely and thank them for their time. If you asked to reserve and item, and find you cannot afford to buy it this month - let the buyer know ASAP so they can release the item for other potential customers. If you are not interested in a trade, just say no. You don't have to say "your shop is so ugly I couldn't find anything I want". Just say "thank you, but I am unable to accept at this time" (I repeat this phrase, make it your mantra).
And now if you excuse me, I have to start smoking.