He's saying he only accepts +8 because he wants the most money from you, or because he wants you to send it to a dummy account and then he'll transfer it again to the hacking account (this can't be done if there are binds). This protects him from you knowing what his hacking account is, since he probably has like $100 worth of runes on it he can't risk it getting banned.
There is no technical limitation that prevents him from upgrading a +9 to +14 to +15. Only proof I can offer is that some guy apparently had his +10 enhanced to +15 by another seller.