The fact that the LL owns the property does not 'trump' the rights of the tenant. Both have legal rights and there are processes for both to give notice that have to be stuck to.
When a property owner becomes a LL, they know, or should know their obligations re tenants and the issues of selling. They should know that it isn't possible for a LL to quickly decide to sell and offload a property with a tenant in it. The fact they own the property is not the issue, but the fact they have a tenant with legal rights is the issue....and if they aren't prepared to accept at, they shouldn't rent.
So a tenant has a contract. Contracts don't just expire, as people might imagine. A 6 month contract doesn't mean the tenant needs to be gone in 6 months...it is a starting point. A LL needs to give notice according to the law, and the contract establishes when the first point notice can be given and how much must be given. So at the moment, 4 months notice can be given once a contract is at the right point for that to be acceptable....in a 6 month contract, that would probably be only after 2 months have lapsed. In a years contract, it would probably be at the 8 month point. However, if notice isn't given, the contract will become periodic and rollover and that notice of 4 months (or whatever is legal at that point) will be required.
It is not unreasonable for the LL. Just because they own the property doesn't mean the tenant who pays for their home should be able to be sent packing within unspecified short time frames. You pay rent for a home and a level of security.
If a tenant does not vacate within the notice period, then a LL can start legal proceedings to evict. Again, there are timescales and procedures which must be followed and can finally result in a court order for bailiffs to evict. It can take a year. Most tenants do not need evicting and vacate when given notice, but some won't....sometimes becaue they don't have anywhere else to go.
So, back to selling......solicitors always tell buyers not to look at or offer on tenanted properties unles they are LLs wanting to let with the current tenant. The reason not to view or offer is that there is no way of knowing when a property will actually be vacant. A mortgage company will not release finds and exchange cannot happen until the property is vacant. It is impossible to know if a tenant will vacate or if they will still be there in a year.
The problem is some LLs are greedy. They hope to earn rent whilst marketing and selling the property. In reality, LLs need to give notice and wait until their property is vacant X before marketing and selling. That means there will be a period with no rent.....but they should know that when they choose to be a LL and have to accept that void period as just one of the parts of being a LL. Some though do try to market and sell with tenants in situ. Sometimes they haven't even given formal notice (currently 4 months) and so the naive buyer who ignores solicitor advice and views and offers can then find that they are looking at notice not having been issued, 4 months still being needed, plus the uncertainty of whether a tenant will go.
And then there are lots of threads on MN with buyers feeling cross that tenants 'won't go'. These people don't realise that the tenants are fully entitled and legally protected in being in the property right until the end of their notice period. They are fully legally entitled to not even be able to delivered notice until their contract allows it. And then if they choose not to vacate,not hey are fully entitled to the process of law about eviction which can take months. It is no good buyers getting cross with tenants who are simply having the legal rights they are entitled to.
The people they should feel cross with are the LL owners who are greedily marketing tenanted properties and also with themselves for not gaining a proper understanding if the issues with tenanted properties.
Quite simply, don't view or offer in tenanted properties unless you want to be a LL to those tenants. And if you're a LL, don't try to market the property until it is vacant.