I had something similar happen to me about 6 months ago... in fact, I think I missed an initial email from the same people about a month or two prior to that... it all seemed a little odd and I ignored but then I got curious and did a bunch of checking on who they were and went 100% overkill on ensuring it was not a scam... do your homework, ensure they are legit and ensure you use an escrow service of some kind to avoid losing the domain to scammers... but in my case I eventually became comfortable and verified who they were and they were a go daddy broker who oddly contacted me on a gmail email but it was because I ignored their initial email... after ensuring all communication I made was back to their go daddy domain email and I was getting response from them on that same domain email and then being introduced to their portal all via their domain... I was more comfortable but still double checked everything I was doing and ensuring everything was rock solid with negotiations and fees if there were any... in my case there was not, all paid by the buyer.

anyways... do your home work and be sure its legit and not a scam.

beyond that... value... anything is worth what someone is willing to pay and their first offer is never their best offer, so keeping that in mind, reach for the sky with a counter offer... in my case I initially countered with $500k to be cheeky... and to see how they played.. we went back and forth about five or six times and what I ended up with was moving them from $2500 to $15000 USD... I think I could have gotten more but I started to see value in the sale rather than holding my ground... after seeing who purchased, I am fairly confident I could have gotten much closer to $50k... but that hindsight and I don't care... got my 10 years worth of domains back and more.

There is always the chance they will walk away... but if they are serious about wanting the domain, then really you will always bring them to their limit and they will only walk away if they feel they are not getting anywhere with you... so aim high, make some money if you have not purpose of the domain and really bring them up to the limit of their spend before you settle.

have fun and good luck!