I have private proxies (about 15) and I don't use them. I just use the ones from the sources I mentioned, which are all free.
Free are public, which means you, me, and your grandmother have access to them. They don't tend to be around for long, so you need to get new ones every day.
Private means only you have access to them. But you have to pay or them. You usually pay for them for a month, and so they last for 30 days, then you get new ones (usually)
Your IP is protected with either kind of proxy, but you are still using bandwidth. SB can be a bandwidth hog, and you might hit your ISP's usage cap. I recently switched from comcast to comcast business class because it removes the 250bg/month limit that all residential customers have. and its the same cost! ($59/month)
every kind of link is spam in a way (at least from marketers), and big G knows about all of them. just diversify your links and you can still use SB on your main site. if it was that easy to get sandboxed, more people would be targeting them competition with SB blasts.