If it is complete and all working, I'd say around $200-$250. But is some places it might bring $400-$450 if the right buyer comes along. It is like buying a used car, it is worth different amounts to different people. If you like it and plan on keeping it then paying a somewhat high price is ok. But if you are buying it to resell you really need to get it at a low price. A lot of the folks here on NLG see older used slot machines for sale all the time, can often pick one up for low price, so aren't as eager to overpay as the average person is.
If you do take a serious interest in it I'd ask them to let you open the main door, look around inside for water damage (from a flood or being left out in the rain for weeks - hey, it happens) and check the main circuit board for battery leakage damage. Some rust and oxidation on the metal chassis inside can be expected, these things are a few decades old. You just don't to find any unhappy surprises when you get it home. Also, it has a bill validator but it may not have been updated to accept the new bills in use now (with big head portraits), you could ask the seller if it accepts the new paper money or not. Some BV's can be software updated, some can not. May not be a deal-breaker, but something to be aware of.