My suggestion is leave the "under construction" page up until you have a very clear plan how you're going to develop it (consider getting legal assistance before doing so) - while that may seem counter-intuitive, less in this instance is likely better...
On a related note, blindly asserting TM rights for an acronym may work against you in the longrun in a UDRP defense, and possibly limit future development / lower resale value, if it's not well thought through beforehand.
The company (or could be a scammer scaring you into selling cheap) approached you - be sure to keep all email correspondence, phone records, etc regarding their intention of purchase...
If they contact you again, and assuming you choose to reply back, do not acknowledge their TM or any other details; ignore extraneous questions (rude, but reason is they'll use your answers against you later! Again less is often better) - stick to discussing price only.
A UDRP is not the end of the world ... best to get professional legal assistance, if you believe an UDRP filing is imminent or occurs - there are some important details (that most respondents ignore / don't know about) that can make a huge difference when responding to a UDRP - often the difference between winning and losing.
Ron