That seller might be willing to accept that contingency provided you also include a clause that allows him/her to continue to market that property until you waive that contingency. Also, keep in mind that there are plenty of other ways to accomplish what you want without necessarily getting the seller to accept that specific contingency.
Additionally, you asked several times what the seller has to lose, and I'll tell you. The longer his/her property continues to rot on the market, the more s/he could watch more equity continue to drain out of his/her property every month in a declining market. For example, if the current market value (CMV) of a property is $180K, property values have declined 33% over the past year, and the property values in that market are still declining, then in 6 months that CMV of that property will be $150K. In other words, that property will have lost $5K/month of equity assuming the current conditions will continue to persist 6 months later.
If you're determined to buy the property, but you'd like to do it only after you've sold your current one, then you could make an offer to buy the property with seller financing or a lease-purchase, and you could refinance him/her out or exercise your option respectively. This way the seller knows that you're serious about buying the property, but you need more time to complete that transaction.