I have 2 different Luke Farmboy Lightsabers, both are original and basically 40yrs old. And one is noticeably similar in appearance to the one you purchased on ebay. It has a very thin tip, thinner type handle, and the shaft is oval (I also have a 40yr old Vader LS that is molded the same, so both are version 1 I believe).
In comparison, my other Luke Farmboy Lightsaber (version 2) has a thicker tip and handle, and round heavier shaft. So it is possible that the one you bought could be real. It also depends what you paid for it. Usually, if it's truly "vintage" the seller will desire a much higher price.
However, one must also be very careful when purchasing on ebay. Some sellers will say and do whatever just to make money. Granted there are probably a few that are truly ignorant, and are only selling description by what they were told. Yet it tends to be "buyer beware" with ebay.