not everyone ranks rarity first. In any event, when a shirt is launched beginning of the season, its not yet "rare" mah, but most of us still get them (especially if you support a particular club etc).
im with Faiz on his ranking as I also mostly buy jerseys to wear, so size is important factor. Most of the time the keyword search in ebay i would put the size too haha, for example "Liverpool XL" etc.
Then secondly i would rank the club / countries. No disrespect any fans, but most of us do support one club or another and there are some clubs (like those with intense rivalry) that we will not buy but will still admire from afar.

Then pricing is important too especially for online transactions. Have to research which is cheaper / more convenient / shorter waiting time etc. Sometimes price can be deceiving. You may see it as expensive now, and want to wait for it to drop to off-season price, but that is a risky move, cos most of the time by off season you wouldnt find ur kit, or no size, or sapu-ed by others etc..so end up have to pay more at ebay later with hiked-up prices.
Rarity, is really by chance. If u see 1 on ebay or other sights, then it could be a split second decision. Its hard to "plan" for rare jerseys as they are what they are - rare.