The factors involved in determining the market value of a planet are quite vast in their scope (and thus be dismissed as "too broad") but I would provide a short overview for you.
This is a very important concern if the buyer intends to build a colony (of living beings) on the planet. Planets located inside the habitable zone would come with a much higher price tag than those located too close or too far from the parent star.
Also, for research purposes (setting up unmanned research labs on the planet) other factors come into account. For example, some buyers (groups or individuals) might be more interested in a pulsar planet (a planet revolving around a fast rotating neutron star) than a habitable planet. Others might be interested in planets revolving around white or red dwarfs. Last, but not least, planets revolving around a red giant might sell for an exorbitant price, provided that red giant phase of a star is a very limited time (as compared to its total lifespan).
This is another very important factor. Is the planet terrestrial, gas giant, gas dwarf, water world or ice world? In case of not being a gas giant/dwarf, does the planet have an atmosphere? If the planet is terrestrial, what is the composition of the crust? What is the atmospheric composition (in case the planet does host an atmosphere)?
Size can be a tricky thing when determining the planetary price. For small scale buyers, small rocky worlds with large amounts of precious metals (precious being a very subjective term) would be much more valuable than huge gas giants. On the other hand, if it is a gas giant with massive amounts of fuel gases (such as methane or acetylene), it would fetch a better price for civilizations interested in space travel, inter-planetary trade etc.
NOTE: size refers to the mass of the planet, not the volume. Also, most customers would require you to provide a gravity (free fall acceleration rate) index of the planet.
Moons And Ring Systems
Another important issue would be moons. How many moons does the planet host? Are all moons in stable orbits around the planet? If not, are those (unstable) moons drifting towards the planet or away from it? Does any moon allow the building of tourist resort? Does any moon allow habitation and settlement? Does any moon have precious metals? Ring systems would be of special interest to interplanetary tourism companies.
NOTE: In case there are some moons with highly prized features, the seller might want to withhold that moon from the deal and sell it as a completely new deal!
Probably the most important feature for lonely civilizations such as us, humans, who want to discover and communicate with intelligent beings in our neighborhood.
Also, some companies might want to purchase a life-hosting planet in order to experiment panspermia (to take some primitive life forms from one planet/moon and release them on other planets/moons in order to study evolution or provide dinosaur hunting safaris or whatever).
There are several other factors involved in determining the price of the planet. But considering that too broad and TL;DR answers aren't welcome here, I would suffice with the factors listed above.
In the end, remember one last thing: in any market, price is determined by demand.