e-Bay, the world's biggest auction internet site, has stringent rules about ticket touting and administrators are monitoring the situation.
A club spokesman said: "It has been brought to the attention of the club and West Yorkshire Police that tickets purchased for the play-off final against Watford on May 21 are available for sale on e-Bay. These tickets must have been purchased by either season ticket holders or members.
"One of the reasons that the processing of applications took time was that each individual seat detail sold was recorded so that the club could identify the purchaser, if required.
"The club made two tickets available to season ticket holders in the genuine belief that this would benefit them and the club is seeking the co-operation of both e-bay and West Yorkshire Police to identify the tickets that are available so that action can be taken against the purchaser under the relevant laws and/or club rules."
Ticket stubs for United's play-off semi-final clash against Preston are also being sold on e-Bay.
Ticket holders from last Friday's Elland Road clash are on the priority list for any remaining final tickets, and a minority of fans are looking to cash-in.
Leeds reported brisk business again yesterday, although the queues were minimal compared to Wednesday's clamour for tickets.
The north east corner ticket office was open again today for season ticket holders to claim their Cardiff tickets and is open from 10am until 4pm tomorrow.
A club spokesman said: "Business at the club ticket office was steady yesterday, after the early rush, with queueing time being minimal as season ticket holders entered their second day of the priority period.
"Contrary to rumours that were circulating, included specifically those on Radio Aire, the club ticket office was been open all day and no-one was turned away."