Replace old furnace before selling house?

We have an old (1988) induced-draft gas furnace that we've had trouble with on- and off- in our house (we bought the house June 2008). Most recently, we had to replace the ventilation fan motor (no central AC while that was out). Many of the components have been replaced. The AC unit outside is about 12 years old.
We're trying to decide whether or not to replace the furnace. We'll live in the house for perhaps two more years. Putting aside the issue of an economic analysis of limping along with occasional repairs (and the associated inconvenience) vs the expense of getting a new furnace, what are the pros and cons in terms of selling the house in the future?
One guy who came to give us an estimate on a new furnace said that the furnace won't sell the house, the house will sell the house. OTOH, our buyers agent for this house said that there's a good chance prospective buyers would ask us to replace it before they took ownership, so we might as well replace it now.

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