Okay, I see the 2 versions now. The pro one seems to have warranty for a year or 10 births, whichever comes first. It doesn't look the other version has any warranty. There are patch kits available for holes. But likely she won't get as many births out of it as she would the pro version due to the lighter weight vinyl but she would know that so it wouldn't be a surprise if she only got one more birth out of it.
The manufacturers web site doesn't state it's only for one birth from what I can tell. They just advertise their pro one as being heavier duty. And since they sell liners, it implies it can be used for more than one birth. And again, your midwife would know which version you had, and if it wasn't going to be useful to her, she wouldn't be offering to buy it.
I'd probably do around 65% then. I may round to the lower sensible number. So, if $135 was what you paid total including shipping, tax and everything (probably not, I'm just going off the list price on that site), I'd probably sell for $80-$85.
ETA: and that $135 price listed was for the pool without the liner, correct? They had some with liner for $158, right? So the $135 already considers the subtraction of the liner. Because as Jessica mentioned, the liner cost would be something I'd consider as well if purchasing.