If the item is used I will offer my reasoning behind my offer and I only make offers on items I am serious about buying.
And that's really the key. Respecting the seller by offering the reasons for your bid. They can either say "that works, let's deal" or "that doesn't work for me. Thanks for the offer".
There's no reason to be disrespectful to anyone. The seller priced for their reasons, the buyer is offering what they believe is a fair price. If it doesn't work, no harm no foul.