Maryland law, unlike Ohio and even Wisconsin, does not specifically mention leveling bars, just a tow bar. The MD law states, that every full trailer shall be equipped with a tow bar and means of attaching the tow bar to the towing and towed units.
The tow bar and means of attaching the tow bar to the units shall be structurally adequate for the weight drawn; be mounted properly and securely, without excessive slack, but with enough play to allow for universal action of the connection; and have a suitable locking device to prevent accidental separation of the towed and towing vehicles.
The mounting of the trailer hitch of the towing vehicle shall include sufficient reinforcement or bracing of the frame to provide sufficient strength and rigidity to prevent undue distortion of the frame.
Every trailer and semitrailer equipped with a tow bar and any special mobile equipment being towed shall be coupled directly to the frame of the towing vehicle with 1 or more safety chains or cables, and the chains or cables shall be connected to the towed or towing vehicle and to the tow bar to prevent the tow bar from dropping to the ground if it fails.
While it appears that from your description, this particular seller is not big on goodwill or helping customers, by the letter of the MD law, they are not violating the law, but if they tow this trailer into another state where a leveling bar is required and they do not have one, then the person towing the trailer is still in violation of the law in that other state.