Thanks for your question.
In order for an offer to be accepted both you and your wife would have to accept, so of course you need to be involved. You've got to get tough with them, demand to see the terms and conditions of your partner's agreement with them. If the refuse, instruct an agent or surveyor to put a value on the property to give you another professionals idea of whether this is sham marketing.
If it is and your ex-partner will not consent to the dropping of asking price or remarketing with a different estate agent (this time jointly) then you may have to make an application to Court for an order for sale at a more realistic price. You will obviously need evidence of the more appropriate valuation. Surveyors are the best form of independent valuation, they are almost beyond reproach.
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She can instruct an agent but he will only have contractual relations with her - she cannot contract on your behalf without your permission or authority.
If you do not have a key then I think you are going to have to instruct a local solicitor to write to them on your behalf, hopefully this will result in their active engagement and avoid the need for an application. If they don't comply then this may be your only option.
I should also state that this answer assumes there are no children involved.
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