A poor person who is examining a piece a cake and another comes and takes it from him is called evil.” (Kiddushin 59a) All legal systems require that goods be legally transferred and deals become binding only when a formal mode of acquisition is made. In Jewish law - and these laws are discussed in the first chapter of Kiddushin - one acquires movables when we they are physically take them into one's possession[1] whereas land can be acquired by cash, the signing of a legal document or actions demonstrating ownership.
While Jewish law strongly encourages competition, to swoop in when another is just about to acquire an object or close a deal is strongly frowned upon - we do not issue the moniker evil lightly - though legally there is little one can do to prevent such[2].  
Commentaries explain such applies only when the deal is just about done but the “paperwork” is not yet complete. But if negotiations are ongoing any and all can enter the fray. Thus if a seller has accepted an offer on a house but has yet to sign off on the deal it would be prohibited for someone to offer an extra $50,000 to get that home. Of course there is nothing untoward about having an open bidding process hoping that people will bid against another. 
One can gain insight into the nature of the prohibition by analyzing a debate between Rashi and the Tosafists. Rashi claims that the prohibition of interloping (also) applies to an object which is hefker, an ownerless object not attainable elsewhere whereas Tosafot disagrees, specifically exempting hefker from this prohibition. If one notices a lost object first but his friend is a faster runner, according to Rashi this would not be the time for a race to the object whereas Tosafot argues that whoever actually picks it up first gets to keep it. 
Rashi views the prohibition of interloping from the perspective of the victim. To snatch away something hefker which, by definition, cannot be acquired elsewhere reflects a serious moral failing. How dare one take away the anticipated gain of another? Perhaps it is for this reason that the prohibition is framed in terms of “a poor person” highlighting the impact on the victim. 
Tosafot (Kiddushin 59a s.v. ani) disagrees claiming the prohibition applies only when the “poor person” wants to rent or buy something and another rushes in to buy that particular object. “Why does he go after that which his friend has put in effort, let him go and do business elsewhere?” But if the object is hefker i.e. is unavailable elsewhere “if he does not acquire this what can he acquire?” It is clear that Tosafot sees the prohibition as one relating to the interloper. He should not take advantage of the efforts of others if he could relatively easily go and get another similar object. But if none can be had what can we expect? Such is the nature of business that when there is only one object a deal is not a deal until finalized. 
Of course such an approach is even more hurtful to the “poor”- and Rashi thus prohibits it - but Tosafot argues such is the nature of business. 
While Tosafot claims that there is no prohibition of interloping on ownerless objects he agrees that it is prohibited to interfere with another’s livelihood. A hefker object does not yet belong to anyone but to actually take away ongoing business of another would be prohibited. One would not be allowed Tosafot explains to try to hire oneself for a job occupied by another - even if one is may be more qualified for the job [3]
Similarly “stealing” clients of others is not sanctioned in Jewish law despite its widespread practice in the business world. One may advertise heavily hoping clients will take up your offer of services but it is the client who must make the first move. 
Interestingly there is greater leeway for enticing an employee of another firm to join yours. While undoubtedly this will impact on your competitor the impact is less - it is generally easier to find a replacement worker than a new client, and any damages are indirect. Stealing a client has a direct impact on revenues whereas stealing workers does so only indirectly and perhaps not at all. Here too blanket dispensation is not given and please G-d we will have an opportunity to discuss this further when we reach the second chapter of Bava Batra. 
[1] Why goods cannot be acquired by money alone is a subject we will leave for a future post. 
[2] The fact that a court may not be able to address the situation does not change the fact that the person themselves should return the object even, Rav Moshe Feinstein (Choshen Mishpat 1:60) argues, if one interloped unknowingly. In the case Rav Moshe dealt with an applicant for a job who was under the impression that the employer would contact him, whereas the employer thought the applicant would call if interested in the job. The communication gap lead the job being offered to another person and when all realized what had happened Rav Moshe ruled the second person should not take the job instead letting the original applicant have the job.
[3] Whether an employer can fire a competent worker in order to hire an excellent one is a matter of dispute with most disagreeing with Rav Moshe Feinstein who prohibited such. While an employer may not be forced to retain an employee a third party has no right to suggest such.