Middle voice ἀποδίδομαι I sell

Let's contrast ἀπο-δίδομαι, ἀπο-δώσομαι, ἀπ-εδόμην,....(trans) "I sell"
Lets otrstwith ἀπ-όλλυμαι, ἀπ-ολοῦμαι, ἀπ-ωλόμην, ...., .... (intr) "I perish, I die"

What's the difference? They are both 2nd aorist middle voice in form, active in meaning, and their 1st person singular forms look identical! But...

The stem of ἀπεδόμην is ἀπεδο. We can and do attach the personal endings directly to that stem.
Note: By stem I mean the combination: compound prefix, past tense marker and base stem. The stem of ἀπωλόμην is ἀπωλ. We have added a vowel (thematic vowel) to this stem before being able to attach the personal endings.
But how do we know that ἀπ-ωλόμην is normal, thematic 2nd aorist whereas ἀπ-εδόμην is a-thematic 2nd aorist?
Because ἀπ-εδόμην is the completely regular middle form of the a-thematic 2nd aorist active of ἀπο-δίδωμι. Its only irregular forms in the aorist tense complex are aorist indicative active 1st, 2nd and 3rd person singular. For some reason those 3 mix past tense augment with perfect endings, just to make things more interesting:
 
  perfect active     aorist active     aorist middle    pluperfect mid/pass   imperfect mid/pass 
I have given
I gave
I had someone give
I had been given
I was given
 δέδω- κα  [ἔδω- κα]    ἐδό-μην  ἐδεδό- μην  ἐδεδό- μην
 δέδω- κας  [ἔδω- κας]    ἔδου (contraction!)   ἐδέδο- σο   ἐδέδο- σο
 δέδω- κε  [ἔδω- κε]    ἔδο- το ἐδέδο- το ἐδέδο- το
 δεδώ- καμεν  ἔδο- μεν   ἐδό- μεθα ἐδεδό- μεθα ἐδεδό- μεθα
 δεδώ- κατε  ἔδο- τε    ἔδο- στε ἐδέδο- σθε ἐδέδο- σθε
 δεδώ- κασιν  ἔδο- σαν    ἔδο- ντο ἐδέδο- ντο ἐδέδο- ντο
 
It is the only verb in the Greek language that has an athematic middle ό-μην ending. In the past, imperfect (mid/pass), pluperfect (mid/pass) as well as aorist indicative middle.
stem vowel ό + personal ending -μην

All the other -όμην endings are:
thematic vowel ό + personal ending -μην
.

Compare:

 Aorist Indicative Middle 
sold Who? perished, died
a-thematic thematic
ἀπ εδό μην I ἀπ ωλ ό μην
ἀπ έδου you (sg) ἀπ ώλ ου*
ἀπ έδο το  he, she, it   ἀπ ώλ ε το
ἀπ εδό μεθα we ἀπ ωλ ό μεθα
ἀπ έδο σθε you (pl) ἀπ ώλ ε σθε
ἀπ έδο ντο they ἀπ ώλ ο ντο
 
*
The 2nd person singular personal ending should of course be -σο but for some reason in endings an σ between 2 vowels gets dropped, and contraction takes place between the thematic vowel and the ending. Since both εο and εο both contract to ου, 2nd person:
"you sold" becomes ἀπέδου.
"you perished" becomes ἀπώλου.
The look the same, but they are of different formation.
 
 
 Aorist Subjunctive Middle 
The normal subjunctive endings -ωμαι, -ῃ, -ηται, -ωμεθα, -ησθε, -ηνται are added directly to the stem of both thematic and a-thematic verbs. But.... Exactly as happens with thematic verbs whose stem ends in ό-, contractions take place:
o + ω becomes ω
o + η becomes ω
o + ῃ becomes οι (proper, clear diphthong)
but ῳ in athematic δίδωμι
Which can only mean one thing:
In the subjunctive (aorist, as well as present), δίδωμι uses the long vowel grade ω and consequently the following contractions take place:
ω + ω stays        ω
ω + η  becomes ω
ω + ῃ  becomes ῳ
so that ... ἵνα (ἵνα μὴ)
Who? may hire may sell may not die
contract verb in -o a-thematic thematic
stem ending stem ending stem ending
  I μισθ ῶμαι ἀπό δ ῶμαι ἀπ όλ ωμαι
  you (sg) μισθ οῖ ἀπό δ ῷ ἀπ όλ ῃ
   he,she,it   μισθ ῶται ἀπό δ ῶται ἀπ όλ ηται
  we μισθ ώμεθα ἀπο δ ώμεθα ἀπ ολ ώμεθα
  you (pl) μισθ ῶσθε ἀπό δ ῶσθε ἀπ όλ ησθε
  they μισθ ῶνται ἀπό δ ῶνται ἀπ όλ ωνται
 
Why the circumflex on the contracted verb forms, but not on the others?
Because the stem vowel and the thematic vowel have contracted but still count as 2 syllables (in the present case, antepenult and penult). And since any contraction is long it carries a circumflex, provided it is not the antepenult, like -ώμεθα (the we-form).
For reasons of pronunciation circumflexes (voice drawn out, going up and then down again) only work on ultimas or penults (only if the ultima is short).
Whereas the stem of "I perished", before adding the subjunctive endings, is: ἀπ όλ.
No end vowel, no contraction, no stress on penult, no circumflex!
The stress stays where it is unless forced to move, as it does for the we-form -ώμεθα.  
 
 Aorist Optative Middle 
The normal optative endings -οίμην -οιο -οιτο -οίμεθα -οισθε -οιντο are added directly to the stem of both thematic and a-thematic verbs. But.... Exactly as happens with thematic verbs whose stem ends in -α, -ε or in -ο like here the stem ἀπο δο (of aorist middle ἀπ εδόμην, contractions take place:
But since ο + οι stays οι the circumflex on the stressed contractions on the penult-looking combination antepenult + penult, if the ultima is short.
so that ... ἵνα (ἵνα μὴ)
Who? may hire may sell may not die
contract verb in -o a-thematic thematic
  I μισθ ῶμαι ἀπό δ ῶμαι ἀπ όλ ωμαι
  you (sg) μισθ οῖ ἀπό δ ῷ ἀπ όλ ῃ
   he,she,it   μισθ ῶται ἀπό δ ῶται ἀπ όλ ηται
  we μισθ ώμεθα ἀπο δ ώμεθα ἀπ ολ ώμεθα
  you (pl) μισθ ῶσθε ἀπό δ ῶσθε ἀπ όλ ησθε
  they μισθ ῶνται ἀπό δ ῶνται ἀπ όλ ωνται
 
Why the circumflex on the contracted verb forms, but not on the others?
Because the stem vowel and the thematic vowel have contracted but still count as 2 syllables (in the present case, antepenult and penult). And since any contraction is long it carries a circumflex, provided it is not the antepenult, like -ώμεθα (the we-form).
For reasons of pronunciation circumflexes (voice drawn out, going up and then down again) only work on ultimas or penults (only if the ultima is short).
Whereas the stem of "I perished", before adding the subjunctive endings, is: ἀπ όλ.
No end vowel, no contraction, no stress on penult, no circumflex!
The stress stays where it is unless forced to move, as it does for the we-form -ώμεθα.