If the seller is asking for your opinion I would give him an honest estimate as to what the item is worth based on condition and desirability etc. THEN I would tell him what I would be willing to pay. This way,the decision of course is his/hers,and....because of your honesty(and maybe even laziness) he may just accept your offer.
In my business I find that this is most effective. I simply say:"Look,this item could get anywhere between say... $200-$600 at auction(depending on who is there,and of course I would also add that after auction fees,that they will get less than the final hammer price),I then say:"I could afford to pay.....(offer what you are comfortable with:$250 or $350..whatever)." I have had many offers accepted when I have come right out and been honest with the seller, and I do start a bit under what I am really willing to pay so that there is some bargaining room.