yes it is the unfortunate part of renting the landlord/agents don't want the property sat empty when you leave so they make your life a nightmare bringing complete strangers round your house when you not there. They have to give you warning of a viewing via letter or phone, it is usually a month they can bring people round because it's a months notice usually, that why you should not tell them until the last day possible so they usually only have a month i.e. dont tell them at the start of the month if you have upto the end of the month to tell them because they take advantage.
Read the tenancy agreement to see what it says, when they can start viewings and what notice they have to give you. And make sure they give you notice other wise you can complain and refer they to the agreement, you could also claim you are unwell, housing confidential paperwork regards your job, dealing with a death in the family, or make the house look like a tip and they might not bring a person to view again until you have moved, if they complain it is untidy say you are packing and it will be like this until you move. They wouldn't want to lose interested parties so they might then wait.
I usually pay for an extra months rent just so I can live out my last month in peace.
I have never had the issue with the house being sold though just re rented, I was once told there had to be an offical break between and house being rented and sold for legal reasons, like a 1 months break between. Maybe this is something you should look into I was told this by an estate agent once but have failed to confirm the fact.