[1] Actually, as Stephanus of Byzantium and others noted, there were two cities called Abdera: one, the present-day Avdira, in northern Greece (Barrington Atlas map 51 grid D3; RE I.22-23; OCD(4) s.v.); the other, the present-day Adra, in southern Spain (Barrington Atlas map 27 grid B5). But the idea of Abdera as 'the sea' is unintelligible.
[2] Similarly glossed in the Etymologicum Magnum and the Lexicon Ambrosianum. An "Abderite" could also signify someone generically stupid: see LSJ s.v. *)abdhri/ths (web address 1 below).
[3] These six city-names (whether spelled with an eta, like Abdera, or epsilon-iota; there would have been little difference in pronounciation by the time of the Suda) generate an ethnikon in the same way. See also delta 1318, sigma 977. This part of the entry, Adler reports, is lacking in mss A (= Parisinus 2625), G (= Parisinus 2623), and T (= Vaticanus 881).