The risk is primarily that they won't pay and won't leave forcing you to evict them at great expense of time and money. During this process the "renters" may damage the property adding to your woes.
Now, if the seller only needs a couple of days to clear out their belongings to move to the new home they are purchasing, the risk goes down. Other ways to lower your risk is to treat them like any other tenant; obtain a credit report, collect first, last & damage deposit, do a move in inspection do determine the condition they are to return the home to you in and put everything in writing.
If this is for more than a 3 days, you need to consult the landlord tenant law to understand your rights and obligations. Iâ€™ve attached a link with this for you. Best of luck.
If you have not been a landlord before, make sure you get a copy of Truth in Renting so that you know the landlord/tenant laws in NJ.
In my experience, attorneys would rather not see them, but if it is needed, then your surely want an agreement in place to cover the period that the seller remains - and that is a Use and Occupancy agreement that a real estate attorney will draft.
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We use attorneys for all our closings up here in northern NJ......I know you might not be using an attorney down there in southern NJ.
The attorney would hold money in escrow (from the closing) to cover the rental amount - which may be calculated on a per diem basis, depending on the length of time.........also an amount to cover a security deposit.....and an agreement in regard to any repairs that might be needed.
The new owner of the home might have the potential downside, moreso than the seller..........the tenant might decide not to vacate on the agreed upon date.....things might break........but with the agreement written by an attorney, everyone should be protected.