Thank you for making your question to JustAnswer. My name is*****'m an Antiques Expert and I will try to help you with your particular inquiry. Please, don't hesitate to contact me any time or to make follow up questions if needed.
William A. Slaughter (American, 1923-2003) was best known for his Texas landscapes.
Regarding the value of your item, I have just perform a market research of similar sold items as yours. I performed the research based upon the most recent finished auctions and private sells, so the value given is the real market value of your item. This is the only way to know the exact value of your unit.
The value will be in the range of 3,400-4,000$.
If you want to sell the unit I would recommend to sell them on the internet so you can reach more potential buyers. You can also sell the lot locally, Craigslist is free, and a lot of collectors look at the ads on Craiglist.
Keep in mind if you sell the unit to a resell shop, the price will decrease 20-30% from market value.
You can sell the item on Etsy, it's like you put the item in a online shop. You may also check Amazon Market Place. Both places have good reputation, and you can easily sell the items for a good price.
Let me know if I can help you any further, otherwise, please, remember to rate my answer!