Ignore what the estate agent says, it very much is the price. It is always the price. Other factors like the size, appearance, state of repair all boil down to one thing - what price is the house worth.
You say the other person won't agree to a reduced price. Well, if they are coming from the point of view of wanting the most money (as opposed to their reluctance to lower the price being a method of control/abuse), you can always reduce your share of the profit.
For example if the house is worth 200,000 and after fees and mortgage repayments your shared equity is 140,000 or 70,000 each, you could reduce the price by 30,000 and your share by the same amount, i.e. it sells for 170,000, the equity is 110,000, you take 40,000 and he takes 70,000. That way he's not affected by the drop in price.
You might say you shouldn't have to, and to an extent I agree, but it's you that seems to want a faster sale.