First let me say "I am not trying talk you into buying this tank." But if you like the tank don't let the livestock hold you back. You could still take your time with the tank.
If you are looking for a tank that size 125g with the type of equipment that is included it may be worth it just for the equipment.
The rock could be kept in a rubbermaid tub while you take you time to set things up.
You could try to trade the livestock for other equipment or trade in some of the livestock at a LFS for credit. I just ordered some fish from Finnyas Fins so I don't have room for anything big. But if you know what fish he has you could try to post something here to see if anyone is interested. If the person selling is not involved with the STRS or URS he may have corals that haven't propagated their way through the clubs and could be new to many around here. Also, someone who just set up a tank may be interested in the livestock but not the equipment.
The seller may want to sell everything together but you might be able to do the legwork to get rid of the livestock. Just think, if you do the work to sell or trade the livestock for $250-300 or more you could be getting all the equipment for $500-$550 or even less.
Another option is to offer livestock to anyone who is ready, willing, and able to help move a 125g tank and all the supplies. Many here, including myself, would help (if available) WITHOUT the incentive. But a little gratuity is always appreciated.
Last year when I was upgrading I couldn't find a good deal on anything used so I bought all new. Spent about $2000 on tank, stand, equipment, lights, live rock and that is for a 90g.
You probably don't want to be a pest to the seller and keep dropping in with different people but if 2-3 of us that might be interested in the corals could go with you we might be able to claim all the livestock.
Just a few thoughts if you serious about a 125g. Make post here and see what interest there is.
Dave