Yep, Harry makes good points.
In general it's good practice to refrain from selling trees which have not fully rooted out. But, in low-supply, high-demand situations, buyers are annoyed if they are not able to buy that one tree they've been desperately wanting to locate.
Over the years, we fruit enthusiasts develop an eye for discerning the good, strong and healthy trees. A desirable specimen should be firm in the soil and should not be in the state of 'hibernation' induced by being in the same size pot for way too long. But newbies don't yet have that eye and really need to be given good advice on how to care for their trees.
It's difficult for nurserymen to sell fully rooted out trees year-round. Generally potting up occurs in the fall or the spring, and it takes 2 or 3 months for most trees to sufficiently root out. So, if the buyer happens to come during the time of the year that pot-up happens, they will either get lucky and get a large tree just ready to move to a larger pot size, or they will get a tree that was just recently potted up and not fully rooted out. Some nurseries will have overlapping stock to deal with this issue. They up-pot a portion at a time, starting with the larger trees first. But, this doesn't work too well in high-demand, low-supply situations.
One way for the buyer to deal with the supply/demand issue is to stay away from the 'cutting-edge'. With most products, there is a huge price differential between mainstream and cutting-edge models.. the latter of which are obsolete in very short order :-). This is true with computers, cell phones, .. and even plants :-). You can either get a giant - say - Rosigold for cheap (because it's not the current fad) or a tiny Mahachanok for a lot of $$ :-).