This may not be very reliable, but the following endpoint will give you different responses based on the availability of a Skype username: https://login.skype.com/json/validator. Here are two examples of (at the time of this writing) an unavailable and available username:
# Request (unavailable):
curl -iX POST -H" Application/json" https://login.skype.com/json/validator?new_username=borist
# Response:
{
"status": 406,
"status_text": "valid",
"data": {
"markup": "Skype Name not available",
"alternatives": true,
"fieldDetails": "<label>Suggestions<\/label><ul><li><label><input class=\"skypeNameSuggestion\" type=\"radio\" name=\"selectSkypeName\" value=\"borist92\"\/>borist92<\/label> <\/li><li><label><input class=\"skypeNameSuggestion\" type=\"radio\" name=\"selectSkypeName\" value=\"borist176\"\/>borist176<\/label> <\/li><li><label><input class=\"skypeNameSuggestion\" type=\"radio\" name=\"selectSkypeName\" value=\"borist417\"\/>borist417<\/label> <\/li><\/ul>"
}
}
# Request (available)
curl -iX POST -H" Application/json" https://login.skype.com/json/validator?new_username=boris3294a
# Response
{
"status":200,
"status_text":"valid",
"data":{"markup":"",
"alternatives":false,
"fieldDetails":""}
}