Interesting (and hard) question! I'm not the expert here, but I'll venture my best guess. "ver-" generally has the meaning of "to make something be" or "to let it be" or "to change it into". It of course then also changes the word into a verb.
E.g.: "oud" - 'old' but "verouderen" - 'to age'; "huis" - 'house' but "verhuizen" - 'to move / relocate'; "branden" - 'to burn' but "verbrand" - 'burn down / incinerate'; "engels" - 'English' but "verengelsen" - 'to become English / Anglicize'.
So taking "kopen" - 'to buy' would mean that "verkopen" is 'to make something be bought' or simply 'to sell'.
The closest I can come to this in English is 'en-' or 'em-' (which is 'en-' with sound drift before certain explosive consonants like 'b' and 'p').
E.g.: 'embox' (to make it be in a box); 'enarmour' (to clothe in armour); 'embronze' (to make it bronze); 'encourage' (to let someone have courage); 'enchain' (to make it be in chains).
I hope this helps. If there are better answers, please share!