You can have negative feedback on eBay but still show a 100% feedback rating. You just need to have enough positives per negative to cause the rating to round up to 100%. Here's more detail on how it works:

eBay calculates your percentage by dividing the number of positives by the sum of positives and negatives, and then rounding to the tenths place. It uses only unique feedback received in the last 12 months.

Since feedback is rounded up to the tenths place, you just need to get to 99.95000% or greater and the rating will be rounded up to 100%. This means you need at least 1999 positive feedbacks per negative to keep a 100% rating, or a ratio of 1999:1 or better.

Examples

Here are some mathematical examples to demonstrate this:

1700 Positives, 1 Negative
1700 / (1700 + 1)
1700 / 1701
0.9994121
99.94121%
99.9%
1980 Positives, 1 Negative
1980 / (1980 + 1)
1980 / 1981
0.9994952
99.94952%
99.9%
2004 Positives, 1 Negative
2004 / (2004 + 1)
2004 / 2005
0.9995012
99.95012%
100%

Testing

To test this, I wrote a script to scrape eBay for random sellers and store their positives, negatives, and percentage in a database. It didn't take long before I found sellers that had 1 or more negatives, but still a 100% feedback rating.