I agree with Watermelons. It is usually best to not give medicine unless it has been specifically recommended to you by a vet. However, if money is an issue, vet care can't be sought, and/or you're willing to take a risk, you can use meds bought at pet or livestock stores. We have to be flexible sometimes because some people's financial situations may be different. Ivermectin is not the first choice for intestinal parasites, and piperazine is safe. I am glad you know not to give ivermectin to your bun. Make sure to check which piperazine type you have, as the table I found has two different ones, with drastically different doses.
http://www.medirabbit.com/EN/GI_diseases/Parasitic_diseases/Nem_gen/Nematodes.htm
http://www.medirabbit.com/Safe_medication/Anti_parasitics/safe_antiworms.htm
This is a good website both in terms of helping you understand the risks of giving it w/o direct vet supervision and for what potentially dangerous side effects to look for.
http://www.petplace.com/drug-library/piperazine-pipa-tabs/page1.aspx
Make sure to monitor for side effects/overdose effects, and get the bun to an emergency clinic ASAP if they occur. Remember also that administering medication to pets can sometimes be perceived as "practicing vet medicine w/o a license" and you could be brought up on charges. The potential risks are pretty awful--bunny could require days of expensive medical treatment, bunny could die, you could be in trouble--and you have to weigh the risks and benefits. Antiparasitics are pretty serious drugs because they mostly act on the nervous system.
For people who have livestock, the mindset is a bit different--they are more concerned about population health and a 1% rate of adverse effects isn't a big deal if you have 100 cows and you lose one. It's better than losing a larger number due to the disease spreading in the herd. Livestock are also treated differently legally in terms of what a non-veterinarian can and cannot do to them, I think (but don't quote me on that). One of the great things we have on this board is a combination of both mindsets, and we pet rabbit owners can learn a lot from people who own rabbitries because they have years of information passed down, especially about treating ailments with less expense and without direct vet oversight. It can be tempting to do what they do with our pet bunnies in an effort to save money and time, but it often isn't the best option. It is a grey area and everyone has to choose what is right for them. I only will give probiotics, simethicone, and ibuprofen or aspirin at home, because those are nearly harmless (ibuprofen/aspirin being the exception that does require a bit of care).
Based on my own experience, it's a bad idea to use any meds you bought outside of the species it was marketed for unless directed by a vet. I had a horrible experience with ivermectin (and a Dutch bunny, didn't know at the time about their sensitivity) that haunts me to this day. I have a lot of education and knowledge of pharmaceuticals and vet care, but I have no certifications to practice medicine on people or animals, and I don't think it is ever worth the risk. Just because something is sold over-the-counter does not mean it is ok to use without a vet's supervision, either in the species it was designed for or in another species. A lot of drugs sold over-the-counter for humans can have some pretty bad effects on us as well! That said, there are some people whose ideas of risk and benefit are very different from my own. In fact, you can find people who recommend treating yourself with bicillin that you buy at tractor supply co for infections!
Here's the TL;DR version. Don't do it.
