How to get difference in value over 15min interval?
Just group your @LatestFifteenMinTickets table by DomainId and take the difference between MIN(SoldTickets) and MAX(SoldTickets). That will give you the number of sold tickets for each domain within the period collected in your table variable:

SELECT
  DomainId,
  SoldToday         = MAX(SoldTickets),
  SoldSince15MinAgo = MAX(SoldTickets) - MIN(SoldTickets)
FROM
  @LatestFifteenMinTickets
GROUP BY
  DomainId
;


Unless SoldTickets can decrease as well as increase (can tickets be returned?), in which case, rather than the minimum and maximum, you should probably take the first and last value. But in order to be able to do that, you should have the Date attribute in your table variable as well. Then the calculation could be organised using the analytic functions FIRST_VALUE and LAST_VALUE:

SELECT DISTINCT
  DomainId,
  SoldToday         = LAST_VALUE(SoldTickets) OVER (PARTITION BY DomainId ORDER BY [Date] ASC ROWS BETWEEN CURRENT ROW AND UNBOUNDED FOLLOWING),
  SoldSince15MinAgo = LAST_VALUE(SoldTickets) OVER (PARTITION BY DomainId ORDER BY [Date] ASC ROWS BETWEEN CURRENT ROW AND UNBOUNDED FOLLOWING)
                   - FIRST_VALUE(SoldTickets) OVER (PARTITION BY DomainId ORDER BY [Date] ASC ROWS BETWEEN UNBOUNDED PRECEDING AND CURRENT ROW)
FROM
  @LatestFifteenMinTickets
;