Think of it like this: If it were a house and you chose the paint colors, it's unlikely whatever color you choose to paint the rooms would be exactly what the buyer is looking for. Spend the money on the important things and the "bones" of the plane. Couple things:
1) $8K isn't going to do much other than plop an EFIS in place of some flight instruments, which changes the overall base functionality of your airplane almost zero.
2) If it's a VFR plane, spending the bucks will not return much on the investment for certain things as compared to an IFR plane.
3) Realizing #1 and #2, if you have an old tube transponder and a 760 radio, no GPS, then upgrading those to something relatively modern would be a wise decision just to get offers on the plane and for it to be minimally competitive in the marketplace.
4) If it's an IFR airplane, buyers just expect it to have a WAAS GPS in it.
5) Any crazy home brewed or kludged together electrical systems/devices should be replaced with industry standard.
6) I wouldn't say you "never" recoup avionics expenses, but it's a VERY specific few that do - there are some specific instances you'll be money ahead...but admittedly that is a minority.
Overall everyone is pretty much on the mark. Discount the airplane for taking into account the panel, and let the new buyer upgrade it to exactly how they want. We do a whole lot of that, and it's almost always better when done that way verses trying to upgrade before the sale (except in a few instances like mentioned above).
With all that said, there is something to keep in mind though. If your plane is poorly outfitted and there are a lot of competing planes in the market (not saying that's the case right now - just pointing this out), but all things being equal if you have 2 planes and one has a poor panel verses one with a nice panel both priced closely, it'll be hard to sell the poorly equipped one even with discounts. Everything will sell at a price that it's worth (whatever someone will pay), so it's just a matter of finding that line between making the plane attractive to a potential buyer and wasting money!
I should be telling you to "spend spend spend", but I'd rather you sell the airplane and have the new buyer "spend spend spend"!
Cheers,
Stein