I have a fairly large friend who has done this on more than one occasion for flights to/from Europe. It assures him a comfortable flight having the extra seat and no need to pay much higher business class fares. Never was a problem on the return either.

The only problem was on one flight within the US on the return - twice the GA tried to put someone in the seat next to him (oversell situation). Fortunately his son was with him, who was polite but firm that his Dad had purchased two seats. Worked out OK in the end.

His TA booked his flight so I don't see why it would be a problem to have a TA book it vs. booking directly with the airline.