On our local exchange here in AU, there is a constant wall on both sides that is almost identical, that is constantly moving. When i query them about it, they said it was someone who was marking the market...
Yep. It's a way to manipulate the market. If I have a bunch of Bitcoin and I want to buy at 50, I put up a sell wall at 50.1 and everyone who wants to sell has to sell below that. It doesn't always work though and it's funny to see a sell wall get eaten into because sentiment changed and it didn't get taken down soon enough.