Anything left over at this point is definitely junk. The owners aren't going redeem them, so you've put out money to participate in the auction and/or to purchase the property, and you're not going to get any money back out of them. They haven't been sold at the auction because they have no market value.
I've purchased a few Maryland tax liens, and I've made decent money for the amount invested, but I do better elsewhere. By the time you pay to register for the auction (in many counties), have the interest bid down at the auction, and then only have it invested for a few months, there are a lot of other ways to make money.
Very few houses actually end up being owned by the tax lien holder, because any mortgage company will pay off the lien in order to protect their interest. Not that it can't be done, but statistically, you would have to buy a lot of good liens to end up owning a house. You can increase your odds by buying liens on houses without mortgages, if you have the time and inclination to do that leg-work ahead of time on the 100s of houses you might possibly want to bid on.
Please feel free to correct me if I have anything wrong here. I'm not an expert; I've only participated in 2 or 3 sales. I just found it wasn't worth the hassle for me.