Backlinks are other sites linking to you. To have high rankings in Google, it's important to have a variety of different webpages -- all on topic and relevant to your site -- linking back to you. It's also important that they be from high quality sites.
I would stay away from that type of eBay scam. The links most likely aren't high quality and aren't relevant. The only true way to get other sites to continually link to you is to have great, high quality content that is link-worthy.
You can buy links from high PR websites. They can be useful in your campaign. Do not buy those eBay stuff or anything that offers thousands of links for small bucks. Thousands of links within a very short period of time is not natural and it would hurt your campaign.
If you really have the funds to buy links, buy directly from reputable websites in your niche. Do not buy from link traders. We have done this that is why I can say link buying from reputable websites is really effective.
Buying or selling back links that pass PageRank is in violation of Google's Webmaster Guidelines and can negatively impact a site's ranking in search results. But
not all links.So you should check that before getting backlinks.