HTG doesn't have a good reputation (with LED at least). Those lights use epiwhatever diodes which aren't very efficient, generate a lot of heat. Each round module has 15 3w LEDs. So, a 4-spot will be rated for 180w, but will draw about 120w. You can extrapolate that into the fixture you're looking at (the number of spots it has).
I bought a new one of those direct from China for $150 free shipping. It was a 4-spot with custom spectrum. It grows ok but generates a lot of heat. I would have been better off buying an Area51 RW-75 for $220. It runs with 45 less watts and grows as well, less heat (because it uses better diodes).
In essence, for $300 you could buy an 8-spot direct from China (with custom spectrum). For $440 you could buy an A51 RW-150 and it will grow as well (with fewer watts). You'll get a lifetime warranty as well.
So, $300 for a used one sounds high. These epi-whatever lights don't last a long time. So, I'd be nervous buying a used one for any price. I don't think it's worth more than $150 used. I wouldn't give that much for it. But, that might be a fair price if someone weren't willing to pay for a longer-term investment.
