I just looked at the Price Guide page for the 8129 AT-AT Walker, and the set is literally, all over the place in terms of growth. Up $50, then down $30, then up $40. LOL, I understand why you might be a tad confused.
When data is hard to read, like in this case, I like to look at the "Performance Over Time", which is located right below the pricing and quantity sold charts, as seen below:
As you can see, this set is up 1.33% in the last 6 months, up 28.84% in the last year, and up 123.03% in the last 2 years, where majority of the growth has happened.
Typically, I don't really look at increased in the last 2 years, on a set that has so recently retired, as a set probably had an initial growth period, directly following retirement, and is for short-term investors. Instead, we should look at slightly newer data, for the last 1 year. This gives us a better idea of how the set might perform now. After the initial growth, most sets slow down, and some even fall in value, so its important to know the time period that you are looking at.
Another way for you to determine how a set may be doing in the next year, is by going to the CAGR percentage, which is located at the top of the Price Guide page, as seen below:
The CAGR, or Compound Annual Growth Rate is . If you have more questions, you can read one, or more, of the many blog articles on it in the Blogs Section. Here's a quote from one of the articles that Ed Mack wrote on CAGR...
So, since the CAGR percentage of the 8129 AT-AT Walker is still relatively high, at 32.03%, this set has time to grow, and could likely break the $300 mark by Summer 2014. Once the percentage weigns to the single digits, I think it is a safe time to sell, as you probably can't look for that much more positive change in the value of the set.
Hope this helped!