There are several things that can help you set a selling price. 

1. The market price: you can look on Ğchange and explore what the average price is for the type of good or service you are willing to sell. 
2. The relative price: you can look on Ğchange and check the prices of goods and services that you would like to buy. Then set the price of things you are selling by estimating the value you think it has in comparison to the things you would like to buy. For example, let's imagine you would like to buy some comic books, and sell some jam. If you find that a comic book is being sold for an average price of 10 UDs, and if you think a pot of jam is worth roughly half of a comic book, then you can try to sell the pot of jam for 5 UDs. There is no guarantee that you will find a buyer at this price. Just keep in mind that the value of goods and services is relative (subjective) to each person; what holds a lot of value for you might have less value for person A, or even more value for person B. As a seller, we tend to over estimate the value of what we own. 
3. The sale by auction: if you think that the product or service you are selling is likely to be quickly sought after by a lot of people, you can organise a sale by auction to get the maximum amount of G1 in exchange for it. That way, you don't have to spend too much time conjuring up a fair price for your offering, and instead allow "the market" to decide how much it is willing to pay for it. To make it easy, you can also consider organising a Vickrey auction, where bidders submit their own written bids, without knowing the bids of other people in the auction. The highest bidder wins, but the price paid is the second-highest bid.
4. The free price option: just post "free price" in your ad. It will then be up to the buyer to determine the amount he or she is willing to pay for your product or service. In general, it has been noticed that buyers are more generous than you expect and will tend to offer prices higher than what you would have expected.