Signet of Capture

If I were to use it and capture a skill,would I be able to re-purchase the signet and capture another skill and still keep the previous skill I unlocked?

in a word, yes

skills captured unlocked (you will always b able to use that skill)
and u can buy another capt signet to capture another skill

Once a skill is unlocked, it stays unlocked. Yes, you will be able to repurchase the signet and unlock other skills.

just be sure you don't buy too many signets of capture. Later on you can buy all the skills at the Emberlight Camp, well almost all. You will have to get elites from bosses by skill capping.