If the other company is a member of a cooperative Multiple Listing Service® (MLS®) which your Agent belongs, then they share listing information to all members. For Sale By Owner (FSBO) properties are not listed in MLS® so an agent would be required to contact the Owner to receive the property information and if you are working with your Agent under a buyer agency agreement, your agent may be able to help you purchase a FSBO.
Sometimes if you call on a sign, an listing agent may entice you to work through them “exclusively”. Be aware that the Listing Realtor may be obligated to provide fiduciary duties to you (look after your best interest) and you may be treated as simply a customer. If you have an Agent or REALTOR® that you know and trust, you should always check with them first or simply insist that you will be represented by your REALTOR® and that your REALTOR® will contact them to obtain further information. Almost always the Seller has a commission or fee allocated to the Buyers agent. The only incentive for dealing exclusively is for the listing agent who hopes to be paid the buyer agent portion of the commission also. By insisting on represntation you can ensure that your interests will be best represented and that you agent is paid for his work further enhancing your working relationship with your chosen agent.