First you can't sell a players jersey until they have signed with the NFLPA.
Second one of my friends is the GM of merchandise for a NFL franchise, you have to order jerseys far in advance because they are produced in China. For instance, if a player comes out of nowhere to be a star if you want a mass quantity of jerseys you have to order them before the previous season is over. After that there isn't enough time for production/shipping etc, so they make the rest domestically. The will send out an e-mail to their vendors offering certain players and then the sports stores and teams respectively can place their order. Seeing as how Spencer was a late first rounder and there are more marketable players out there, it may be a while. Because you have to wait for Reebok to produce the higher profile players first. That is why last year there were Romo jerseys after he debuted. They offered a set quantity; however, outside of the pro shop they were very hard to get. The stores sold out of their online stock rapidly and were in heavy back order. I think my girlfriend tried for 6 weeks to get me a Romo jersey for Christmas before giving up and finding something else.