Replace Dyneaudio Tweeters or Sell As Is? Newbie Seeks Advice?

Hi OP!

The diaphragm replacement is your best bet.  You'll sell them for a lot more if they are fully working than for "guess what's wrong" price.

Replacement of tweeters is otherwise a difficult ask, as the crossover is designed for a specific electro-acoustic device.  You can't just go willy-nilly looking at the physical size and get it right. Sometimes you get very lucky though.

I'd suggest that if the diaphragm replacement is not an option you then sell as-is.

Mids and Woofers, definitely stick with OEM or repair OEM,

Tweeters: I meant to get specific recommendation from Madisound, they know equivalent specs/performance.

The crossover has already done it’s job, the same impedance and efficency/sound level is more important, if needed, go a speck higher, not lower.

General joke is that by the time you can afford good stuff, your hearing of highs is likely already reduced.

And, you are selling them don’t forget, just don’t hide the truth from buyer.

Thank you all for such helpful insights! 

DO NOT buy one of the replacements mentioned. All 3 have radically different impedance curves.  They would not act anything like each other with your dyn crossovers.  One of them had a 1mm xmax. No suitable.   Next the ferro fluid issue.  
I have purposely removed ferro fluid from MANY tweeters.  In every case, it lowered fs and sensitivity increased slightly, but other than that, the other tweeter specs remained fairly consistent.  The warning about drying out fluid or worrying that needs to be replaced is valid.  If it really worries you,  thoroughly clean out the gap and either replace the fluid or leave it out completely.  A different fluid should be fine if it is good quality and the original is completely removed.  Or, you can run them without fluid.  Some worry that without fluid that they can damage easier,  yes fluid is for heat dissipation,  but when you remove it,  fs will lower a bit.  If the crossover point is high enough, I would remove it,  if not, I would make sure that you have fluid. 
As far as replacement.  If you can send me a good tweeter, I will spec it for you at no charge and truly help you find a proper replacement and be aware, that may not be easy. I hope this helps,  Tim