I have had my 2001 GS300 for a couple of years now, great car, only having to perform the ususal maintenance. I priced your 99 out on edmunds, selecting the typical options for a 300...No NAV, No HID's, No ML, No Chrome wheels. (Beware of chrome wheels) I get $11,400 for retail. Unless they have documentation of all maintenece, timing belt, water pump, plugs etc, BRAND new tires and this thing is jaw dropping immaculate...$13,000 is way too much for this car. You have to remember most dealers are only going to give KBB "good" value on a trade-in, which on that car is between 7-8k. I can't check KBB b/c their site is down. I do trust edmunds car values. I think you could come up with a lower mile and newer GS for the same money.
Good luck,
Kerry