Gemma,

It will take a lot of feedback to get back to 100% but the reason it is showing as 75% is because you have received so few feedback in the last 12 months.

The feedback percentage is calculated on feedback received over the last 12 months.

To improve your feedback rating you could buy a few cheap things as that will quickly improve the percentage for you.

You could also leave a polite follow up to the negative feedback as buyers will often look at how a seller responds to negative feedback.

Don't be scared to try and list a few things. I would assume you are eligible for 30 free listings a month so even if the item doesn't sell it won't cost you anything in listing fees.