I am going to make the argument here that "Twitter" might be a TM for a social site, but was also a dictionary word hundreds of years prior. There is more than one possible use for this term.
I don't see anything on the site that would be infringing on the social site.
–verb (used without object)
1. to utter a succession of small, tremulous sounds, as a bird.
2. to talk lightly and rapidly, esp. of trivial matters; chatter.
3. to titter; giggle.
4. to tremble with excitement or the like; be in a flutter.
–verb (used with object)
5. to express or utter by twittering.
–noun
6. an act of twittering.
7. a twittering sound.
8. a state of tremulous excitement.
Origin:
1325–75; ME twiteren (v.); akin to G zwitschern
Brad
You don't know what the buyer's motivation is. What a buyer uses the domain for is up to them. However, your website is in no way infringing with the Twitter social site as is. IMO