I'm not sure why you refer to "45 days left before leaving." If you have found a new place and 45 days is based on your plans, you don't risk much by declining to allow entry during that time.
Even if this is considered a breach of contract, the landlords' remedy would be to seek a court order to require you to allow entry; that's expensive and probably not worth the hassle for the landlords. On the other hand, if you have not chosen to leave in 45 days, you have a lease through the end of the year or at least as long as the current landlords own the property -- probably quite a bit longer than 45 days if they don't yet have a buyer.
The oral promise not to sell won't help you; it is superseded by the terms of the lease.
As for allowing an open house, most tenancy agreements have language authorizing the landlord to "show" the property to prospective tenants or purchasers, but I don't read this as authorizing an open house, which typically requires the resident to make the house attractive to buyers and remain away from home for a few hours. I would not assume a duty to cooperate with an open house unless there is more specific language in the lease; even then, the consequences of refusal would probably not be severe. Leases don't generally include liquidated damages for failure to allow the house to be shown.
If you paid a security deposit at the beginning of your tenancy, getting into an argument with the landlords about these issues might result in delay getting your deposit back, as the landlords might claim a right to withhold it. As is often true, in deciding whether to go along with what another party wants or whether to push back, the technicalities of what is or isn't a breach of contract may be less important than the practicalities of who can do what to make the other party's life miserable.
Disclaimer: exchange of information on this site does not create an attorney-client relationship.
A lease can generally provide for a landlord to enter and show the premises to prospective tenants or purchasers, but that has limits. Begin with a review of your lease, and then bring it to an attorney.
Do you want accurate, personalized, legal advice that you can rely on? You will have to hire an attorney, not ask on Avvo. I am not your attorney and am not creating an attorney-client relationship by this post. I am therefore giving only general advice. This advice may not apply to you or your situation; may not take account of all possibilities, and may not match the advice I would give to a client. DO NOT rely on this advice or any other advice on Avvo to make your legal decisions. If you want an answer to a legal question you should retain an attorney who is licensed in your state.
Despite the verbal agreement you reached with your landlord, you might be out of luck subject to any terms contained in your lease. If the landlord's agreement not to sell was material term of your agreement, it should have been put in the written agreement. Notwithstanding, your landlord's right of entry into the premises is subject to limitation. You should refer to your lease for restrictions and any necessary notice requirements before entry can be made.
This "answer" is for information purposes only and is not intended as legal advice or to create an attorney-client relationship.