I have a sears roebuck in wall furnace model 867.736471 and I'm selling the house. The buyer wants me to move the vent from its horizontal position outside to vertically up above the roof line. If I do this will it choke out the furnace and or plow the pilot light out a lot.

Please help

The vent should be installed as per the furnace manufacturer's specifications. You could have liability if you personally make the change and it is not recommended by the manufacturer.
Find the manufacture's specification/owner's manual recommendation.
It is realized that you are in a delicate position here. If what the buyer wants is risky in addition to inefficient, the only suggestion I have is that you consultant a licensed, certified heating expert. Let the expert make the decision. If the experts ok's it let the expert do the installation. If the expert won't do it, notify the buyer in writing of the findings.
You could always offer the buyer a reduction in price, allowing the buyer to make the changes after purchase. All depends on how badly you have to make the sale to this buyer, but making the sale and being sued later is a risk I wouldn't be willing to take.
Was this the recommendation of a home inspector?