How much does everybody's buy rates vary from day to day. Mine can flucuate around ~0-100% a day which seems like a lot but has to be normal. Usually the range varies about 30% but it is common to sell twice as much on a given weekday than the previous day for no apparent reason.

What are your buy rates mine usually are in the 3-6% range

I calculate this by total unique visitors divided by # of sales