It's impossible for us to determine the value of the items for sale based on your description.
So much of the value of used gear relies on how well the gear was maintained and whether or not it is functional "as is."
Reading between the lines, you lack the training and experience to bargain hunt for good deals on used scuba gear. If you buy something with this level of knowledge, it greatly increases the possibility of paying more for an item that it is worth, getting stuck with gear that is not salvageable, and/or having to sink more money into servicing/repair in order to make the gear dive-able.
If you want to consider the purchase of used stuff in the future, I'd recommend either investing the time to acquire the knowledge on your own or taking someone with you who has that knowledge to inspect/test the gear for sale.
FYI, Scubapro equipment warranties are not transferable from the original purchaser to a person who buys the Scubapro item(s) as "used" gear.
Used scuba tanks in good condition can be a great bargain. Before negotiating the sale, you should know the cost of hydrotesting, VIP, and tumbling of a tank in your local area. Rust inside the tank might require tumbling. Lots of used tanks for sale need to hydrotested and VIP'ed before being put into service. Some potential buyers are savvy enough to make the sale contingent upon the tank passing hydrotest and/or VIP.
Used regs can often be bought for less than 50% of the new retail price. Once again, you should know the cost of servicing the reg (overhaul parts + labor + replacement of other parts not usually included in overhaul kits) before buying anything. Some people interested in buying used regs will have a trusted reg tech/dive shop inspect and functionally test the reg prior to making the purchase. Even if this "check up" costs a few bucks, it might be worth paying for it since the servicing could save you from buying a lemon or, at the very least, give you a heads up on any additional costs that you might encounter in the near future.
Used BCDs for sale really should be subjected to comprehensive functional testing. By this, I mean every single part of the BCD should be tested for functionality -- just like in a pre-dive check. Do all of the straps and buckles work OK? Are there any visible rips/tears in the fabric? Are there any holes in pockets? Does the tank camband hold a tank securely? Does the integrated weight system work appropriately? Does the bladder hold air over time? Does the power inflater assembly work appropriately (inflate ON, inflate OFF, exhaust ON, exhaust OFF)? Does oral inflation work? Does each dump valve work properly? Reviewing this list, you'll note that you need a tank with some air in it and a functional reg setup to do the tests. It's worth noting that if any issue is discovered, it makes sense to learn how much it would cost to repair/service/replace the problem part. By the way, all of the above checks presume that the BC fits you correctly and the BC has all of the features that you want.
All this being said, Scubapro has a long track record of manufacturing very good quality scuba gear, a highly regarded customer service network, and an extensive dealer network (which facilitates expediency/convenience in servicing). Those are certainly characteristics that I look for in a gear manufacturer whether I'm considering purchasing new or used gear.
