We have around 15 plants on our balcony which are patented but planning to move overseas. So I was thinking to sell them on gum tree or eBay.
Is that allowed? I did not breed them, just had the plants for 3 years.
You may not propagate patented plants, via cuttings or in any other asexual way, without permission from the patent holder. (You may, however, breed them sexually.) You may not sell multiple plants asexually reproduced from a patented plant, particularly not as part of a business or commercial nursery operation, without permission.
But if you obtained an individual plant legally, from the patent holder or the holders delegate, you can resell the same individual plant, unless you agreed not to do so as a condition of obtaining the plant.
You should be sure that the new owner understands that the plant may not be propagated.
See "How To Avoid Violating Plant Patents" for more detail.