Unless you have a ready and willing buyer, you will need to pay a commission to a sales agent/broker. Commission percentages vary and many brokers have a minimum. (That's why small contracts are more expensive than larger ones if you look at the per point cost). It is also my understanding that you cannot sell your membership without paying off the morgage first if you have Disney financing. (I'm not positive on that, though).
Other costs are negotiable between buyer and seller. The buyer usually pays the closing costs (average between $400-$550). Either you or the buyer will have to pay the maintenance dues for the year. Contracts that have banked points and all the current use year points available are worth more than a contract with borrowed points.
Remember that Disney has the right of first refusal.