It is obviously essential to confirm that the original lease gives the tenant the right to assign (sell) the lease. You should also be aware that the lease may give the landlord some right to vet whether you are an acceptable tenant - for example, to check that you are of equally good financial standing or to require you to provide security (eg a deposit or a guarantee) equal to that provided by the original tenant. The terms of a lease can be open to interpretation if, for example, the lease states that the landlord shall not 'unreasonably' withhold consent to assign.
While the price (if any) you pay the existing tenant will be subject to negotiation, the landlord is unlikely to be prepared to negotiate key terms of an existing lease such as the rent, the permitted use of the premises and any restrictions on alterations. This may give you less flexibility to ensure that the lease suits your requirements.
More broadly, the transaction will involve not just you and the tenant, but also the landlord. This may well lead to extra complication, delays and costs; you are likely to be asked to pay your own, the tenant's and the landlord's legal fees, though this is open to negotiation. There may also be more options as to how the deal is structured - for example, whether you are purchasing the lease or are purchasing the existing tenant's business (including the lease), and whether you are purchasing any extras such as equipment.
Pay particular attention to what liabilities you will be taking on, particularly if you will be responsible at the end of the lease for restoring the premises to their original condition - when the lease was initially granted, rather than when you take over the premises.
In practical terms, you should satisfy yourself as to why the existing tenant is selling, and take advantage of any local knowledge the tenant can provide.
