I believe that as soon as the rebates end, the resale value will go up slightly on used Teslas. It will not be much, but when I purchased mine, it made no sense to buy used unless I could get it for $10k less than new since I got a $2,500 CA and $7,500 federal rebate.
New was $80k, and I could not find a single used Model X under $70k in my area.
If those rebates did not exist, I might have gone used if I found one for $75-77k if I found exactly what I was looking for.
So I believe they will go up slightly but in the grand scheme of things it won't make much of a difference.