No, the used parts market is odd. That's what those parts are selling for on eBay. Not just what people are asking but actually selling for. Used AMD FX series has always been oddly high.
Yeah, it's kind of bizarre that most used FX-8350s are currently selling for upward of $120 on eBay, while it was possible to buy them brand new for well under $100 a few years back. I imagine they are selling to people who are trying to breathe some life into old AM3+ systems with even lower-end processors, but at this point that seems like a bad investment. Even a 4-core, 8-thread i3-10100F will wipe the floor with that processor, and can be found brand new for under $120. Userbenchmark is showing the 10100 as being around 70% faster at lightly-threaded tasks, and over 40% faster for heavily-multithreaded workloads. And an i5-10400F for $150 can more than double the FX-8350's heavily-multithreaded performance. Sure, one would need a new motherboard and RAM for those newer processors, but it seems like a better long-term investment.
The prices for those Asus Sabertooth mobos have also held up surprisingly well, and there seems to be a decent number of them being sold on eBay. There's even less reason to be spending that kind of money on an AM3+ board at this point though, since anyone needing a new motherboard should be looking at the current, much faster budget processors rather than anything that will fit into AM3+.
For anyone looking to sell either of those on eBay, it looks like a really good time to sell though, even if you don't try to get quite that much out of them. If priced around $100 before shipping, they would probably sell super-fast, at least in this market.
As for the HD 6850, it might not be as worth bothering with right now. They seem to be selling for around $20-$40 before shipping is added, and that's not as significant an amount as what you would get for the CPU and MOBO. If you don't have another card aside from your R9 280, it might just be worth keeping it around as a spare, in case your graphics card were to fail or something.
Put a $20 SSD in it and install Windows 10 Home. Since MS lets Windows run in a non-activated free mode now. Why not? If the buyer wants to get rid of the watermark. They can buy it.
Then you have a completed system. Way easier to sell. As now it is accessible to all buyers. Not just tech savvy ones.
Combining the parts might be an option, but adding an SSD would probably be a waste, unless one were planning to install everything into a case and ship a larger, bulkier item. I suspect you would probably also make less profit per item when combined. At the prices these parts are going for, it might just be easier to sell them individually. Combining them might make them more attractive to someone building a system, but it cuts out anyone buying parts to upgrade or repair their existing hardware, and there are probably a lot more people looking to upgrade their AM3+ system at this point than there are those looking to buy a complete system with these older parts.