Depends on your state.
In Hawaii, the manufacture, possession, sale, barter, trade, gift, transfer, or acquisition of detachable ammunition magazines with a capacity in excess of ten rounds which are designed for or capable of use with a pistol is prohibited. However, magazines originally designed to accept more than ten rounds of ammunition which have been modified to accept no more than ten rounds and which are not capable of being readily restored to a capacity of more than ten rounds are allowed.
So, for Hawaii:
1. Manufacture, possession, sale, barter, trade, gift, transfer, and acquisition are self explanatory.
2. Is the magazine designed or capable of use with a pistol?
NO. Then round count does not matter.
Yes, then:
3. Has the magazine been modified to accept no more than ten rounds?
No. Then the mere possession of the magazine is a misdemeanor unless inserted into a pistol, which would then be a class C felony.
YES, then:
4. Does the modification prevent the magazine from being readily restored to a capacity of more than ten rounds? NOTE: no court in Hawaii has ruled with respect to what the term “readily” means. If the Legislature wanted to used “permanently,” it could have done so.
No. Then the mere possession of the magazine is a misdemeanor unless inserted into a pistol, which would then be a class C felony.
With respect to your initial question, assuming they are magazines that are designed or capable of use with a pistol, make the purchase, but make sure they are modified to comply with the law before bringing them into the State.