The original post talked about selling partially used tickets on eBay, so any mention of a risk of getting scammed is out of place here. Unless the OP is a scam artist, there is zero risk that anyone will get ripped off: the seller will accurately represent the number of days on the ticket and the buyer will pay up front before the tickets are sent.
Moreover, it is the buyer who assumes all the risk that Disney will take away the tickets and deny admission, so the OP has no risk there either. Not that there really is a risk, since Disney has absolutely no way of knowing whether a park hopper was purchased on eBay, nor do they care (the case is different with annual passes, which is why they use the biometrics.)
If one person on these boards can document an actual case of Disney saying, "Sure, these park hoppers are valid but you purchased them partially used on eBay so we are confiscating them and kicking you out of the park and calling the police," I will eat my shoe.
Whether selling partially used tickets is legal, ethical, or moral is a debate that rages here constantly with no resolution in sight. However, the OP was asking about the viability of the plan to use a pass and sell the remaining days, and the clear answer to that is: no problem, go for it if you want.