Not sure I understand the second paragraph. Are they saying that if you have used your 90 days up then even though the 180 days have come to an end you cannot re-enter until you have been out of the country for 90 days?
When my 90 days is finished, can I purchase another visa and come back?
No, a tourist visa only allows you 90 full days entry into Turkey out of every 180 full days. Meaning you cannot visit again for another 90 days.
(This rule was fully enforced on the 31st of January 2012)
Always remember it is a maximum of 90 days in every 180 days.
The days not used will 'roll over'. (meaning can roll over to a new visa)
Think 'what is my intended departure date from Turkey' and start counting back over a 180 day period. If you have spent a total of 90 days then you cannot re-enter for another 90 days. If you haven't reached 90 days, then you can return to Turkey for your remaining days.