Encyclopedia Americana 1970

A full set consists of 30 volumes.
You do not mention condition of your set but that will be a big part of the determining factor when looking for value.
I did not find a set of 1970 edition listed for sale but there are several other editions listed and these are probably very close in information as they stated very little was updated/changed during the 68-72 period.
Here are the listings on eBay so you can place a watch on any set to see if it sells. Books are usually very slow sellers and many are listed for months/years before (or if) they sell. Sometimes sets sell faster if condition is good and price is in line.

Please remember that asking prices are just a price a seller would like to receive for their item but only a sold 'identical' item will show the current value as it shows what someone was willing to pay for that item.
www.ebay.com/.../i.html?_from=R40&_trksid=p2499334...

Two sets of 1970 edition have recently sold on eBay but I do not know condition of your set but you can check them out.
First set sold for $100.00 and second set sold for $149.95.

www.ebay.com/.../224017084023?hash=item3428756e77...
www.ebay.com/.../264825862407?hash=item3da8d9f907...

You could try listing your set on sites like Craigslist, Facebook Marketplace, offerup, letgo, etc and even offer free local pick up.
Start your price higher - $150-200 (if condition is very good) and best offer considered.

Leave it listed for several weeks and lower the price if no one makes an offer.
Shipping should be by Media Mail and several boxes.

Here are some listings that you can check out on eBay. As you see from the listings the books are not selling for too much at all. There are some people who are trying to make a few extra dollars by selling books one at a time. www.ebay.com/.../bn_7024752634

Selling sets of books like this is harder online because of the shipping cost to send them off to the buyer. You can try to sell yours on Craigslist or even at a yard sale in your area. The price you'll get for these books won't be a lot.

There are some people who love to do crafts and they actually will buy some of these types of books for a very low price for their crafts. You can try listing these for around $35-50 to start off with and lower the price if you get a good offer. Not a lot of people actually want a full set of books like this in their home because they take up way too much room and all the information is updated online and easier to find.

It is sad that these books are not bringing in more than a few dollars. Online sellers are trying to sell there books in singles to get rid of them. Often the offering prices are discounted due to them sitting on the market to long. Right now a single book starts at $6 plus shipping .

Maybe through a collector it would bring in more, but there are so many for sale, and it's looking like the worth is less. www.ebay.com/.../264218990709?chn=ps&norover=1&mkevt....