Almost as soon as many people decide to purchase a telescope they get this notion. Slow down and relax. Eyepiece upgrading is one of the fastest ways in this hobby to unecessarily lighten your wallet or purse. If you purchase a quality telescope it may come with some pretty decent eyepieces. USE them! And not for just a month or so but put them through their paces for months. How else will you know what you may want or expect an eyepiece to do. If you can get to a star party do so, ask questions, and look through different eyepieces in telescopes that are very SIMILAR to yours in design and focal ratio if at all possible. A telescopes focal ratio, that F/3 to F/20 number, can either make or break an eyepiece as far as performance is considered. An eyepiece that gives flawless images at F/10 might make you seasick when used in a telescope with a focal ratio of F/5. A telescope manufacturer knows this and will usually supply eyepieces that will work fine with their telescope. The eyepieces included have appropriate focal lengths and apparent fields of view with an eye on cost control.

Now back to your specific telescope.
For a dobsonian or other undriven mount telescope you will eventually gravitate toward the widest apparent field of view eyepieces that you can afford. The larger the telescope the more you will need them. Tracking is so much easier when you do not have to constantly nudge the telescope along to keep an object in the field of view. The wider field also enables you to see more of an object rather that just parts of it at a time.

If you have a go-to or computerized tracking mount along with some style of good finder scope your requirements may be different from an observer using a telescope that lacks them. You might not want the ultra wide apparent field of view eyepieces but they are a nice luxury. Many people like to see an object nicely framed in the field of view. So unless you are using higher magnifications or viewing large objects you can get by with a smaller apparent field of view eyepiece.

Observer preferences are most important. Considering eye relief, the distance between your eye and the position of the best image, is very important. Not everyone can hold their head steady unless they feel an eyepiece eyecup touching their eyelashes. Others may need to wear their eyeglasses. If you observe in the winter where it really gets cold then an eyepiece with short eye relief will tend to fog up faster. Being very close to the metal barrel with your face can lead to parts of your skin freezing to the eyepiece. Your first reaction in such a case is to pull back quickly. Either you loose a bit of skin or the telescope follows you along. Not fun.

Do you like the immersive or spacewalk feeling? A large eye lens on the eyepiece can make a difference here even if the apparent field of view is 45 degrees. For real "falling into the telescope views" however eyepieces with apparent fields of view of 68°, 82°, and 100° are available. Some people complain about the wasted field of view that they paid for when they cannot see the inside of the eyepiece field stop or barrel. That is a valid point if it bothers one. Personally when I go to look out a window and can't see the sill it does not bother me. It probably is dusty anyways.

Eyepiece sets or kits. That notion makes me shudder. Sometimes all the eyepieces in a manufacturers line up will perform identically, sometimes not. Several manufacturers market very nice kits at very very nice pocket emptying prices. Good stuff in those kits for sure. But most times a kit is marketed to sell to a wide group of users. If you have a large telescope, where the seeing can greatly affect your range of commonly useful magnifications, or a smaller telescope with a long focal ratio the short focal length eyepieces may sit in the box for years on end. For a telscope with a focal ratio around F/4 a 32mm or longer focal length eyepiece will be of little use. Value enhanced goodies in the kit? To me all those colored filters and lens cloths in the kits are like the giant box of crayons you got with 32 different colors when you were a kid. You only scribbled on the wall with the red and green ones.

So by exhaustively using the eyepieces that you have and trying other types at star parties you should be able to avoid going through many cycles of purchase, use, and sell.
Read the various eyepiece reviews and pay particular attention to the eyepiece attributes that mean the most to you. Also pay particular attention to the focal ratio of the telescopes being used in the review. If you cannot try out other eyepieces rest assured that the ones made by the top end manufacturers will most likely work very well in your telescope. If you are planning to stay in the hobby for a long time then in my humble opinion it is better to save up for a few good eyepieces rather than getting several boxes of mediocre ones. Three eyepieces and a barlow or five eyepieces without a barlow should satisfy most everyones needs.

Dude!