Susan, welcome to Astromart! You have found a great group of enthusiasts here.
1. Ads stay up for around ten days, if I am not mistaken. The TOS states that you may place one ad for an item per day, so I tend to relist an item after about four or five days if I don't get any responses to my ad. That's also a good indication that it may be time to lower the price a bit.
2. Yes, you will get hits on your ad even if you don't pay the extra for a highlighted ad. The main advantage to paying for a highlighted ad is that it stays on the front page, while other ads are bumped down as other ads are placed.
3. The stars are ratings. Once you have made contact with someone regarding an item you wish to buy or sell, you have the ability to rate that person. The usual protocol is that you only rate those with whom you have completed a transaction. Five stars are as high as you can go, and zero is the fewest number of stars that you can give. The number in () that you see next to a person's name indicates the total number of ratings that they have, with the number of stars being the average rating. Use the rating system carefully. Retaliatory ratings are not the norm here, but some people do tend to rate others even if no transaction took place, but the seller took too long to respond to an inquiry. This is discouraged. Besides, it's rude.
Hope this helps!
Doug
Midway, FL