Anything we say here is general information and does not relate in any reliable way to the specific case, whose exact details we do not know. Since the case is not halacha l’ma’aseh (we obviously do not expect woman #1 to sue woman #2), we are free to conduct here a general discussion.[1]
We start with the halachic issue you raise. Many people think that one acquires items in a store by paying for them. However, the halacha is that money is not a valid kinyan for movable objects.[2] Rather, one must perform a physical kinyan on the item, which for items easily picked up is usually hagbaha (lifting the objects). Nevertheless, although woman #1 did physically perform an act of kinyan when she took the items off the shelf, that does not mean that she necessarily acquired the items at that time.
Classical poskim discuss at exactly what point we assume that one who performed such an action intended to acquire the item.[3] However, in the contemporary context you describe, the kinyan is clearly not effective until the buyer lifts the item after paying for it, and certainly not before or when the buyer puts it in the shopping cart. We can be confident about this conjecture because it is very common for someone to put an item in the cart, only to change his mind later and return it to the shelf. If he would have acquired it, he would need the store’s permission to return it. Since buyers do not do this, it is a sign that taking an item off the shelf to put in the cart is simply the first step toward the likely, future purchase of the object. Thus, woman #2 was not stealing from woman #1, as the latter did not yet own the items.
There is a relevant, interesting concept in the gemara[4] called ani hamehapech b’charara that likely applies to this situation. The classic case is when Reuven is in the midst of efforts to acquire something, and Shimon, who enters the scene later, beats Reuven to the acquisition.[5] In such a case, Shimon is called a rasha.[6] (There is a machloket regarding whether there is any binding or practical consequence of that status.[7])
There is a machloket Rishonim[8] if this affront applies only to one who buys and/or rents an item instead of the one who had started the process (Rosh), or even to one who tries to acquire something from hefker (Rashi).[9] The Shulchan Aruch[10] cites both of these opinions. The distinction between buying something and receiving it for free is that receiving for free is generally a unique opportunity, unlike purchases. This distinction could pull us in either direction, depending on whether we focus on the person who tries first to acquire the object or on the one who actually takes it first. If we focus on the former, we can say that in the case of a purchase, the first person does not lose much by not being able to buy at this time, as he can buy later or elsewhere. In contrast, acquiring from hefker is a fleeting opportunity, which he will lose forever, thus making protection of the former’s rights at least as appropriate. The logic of the opinion that ani hamehapech b’charara does not apply to taking from hefker is that in that case, the second person’s actions are understandable because of the rare opportunity that arose for him. This is unlike one who seizes a sale instead of his counterpart who is in the midst of the process. Such a person is acting in a needlessly petty manner, since he should have taken the time to go elsewhere to buy, and this is why ani hamehapech b’charara applies to sales.[11]
The Rama[12] writes that according to the opinion that ani hamehapech b’charara does not apply to hefker, it also does not apply to an item with a unique sales price. The logic here is the same as above: It is not petty for the one who comes second to try to beat the one who came first if the one who came second has no equivalent alternative opportunity to acquire the item. If the event you saw involved such a special sale, then according to this opinion, these special circumstances could arguably save woman #2 from the negative status of ani hamehapech b’charara. On the other hand, the Shach[13] argues and maintains that ani hamehapech b’charara applies to sales even if the price is a uniquely good one.[14]
In truth, we can suggest that what woman #2 did actually did constitute theft, but for a different reason than what you suggested. If woman #1 did not acquire the items, then the store still owned them. It is possible that the store strongly objects to one shopper taking a product from the cart of another, as such actions might cause very unpleasant shopping experiences and could in some cases even lead to shouting matches inside the store. If this is the case, woman #2 can arguably be considered stealing from the store, which does not permit one person to take items from someone else’s cart. However, we would imagine that such an occurrence is uncommon, and it is therefore difficult to determine in advance whether a given store’s objections are strong enough to counteract the general assumption that anyone who brings items to the cashier has a right to buy them.