Answer:
A valid offer to contract can result in a valid and legal contract if the offeree accepts it. In order for the acceptance to be valid it must be made within certain time limits and communicated to the offeror, and also the acceptance must have identical terms to the offer. If the terms vary, it is considered a counter offer.
On the other hand, an invitation to make offers would be like selling something on Ebay so that everyone can see it and those interested can make an offer or bid for it. But if you are selling a signed baseball card, you probably have only 1 of each type and even if you have thousands of people interested, you are going to sell it to only 1 person. The invitation to make offers or negotiate isnot a firm offer, e.g. you do not have to sell 1,000 signed baseball cards to everyone who wanting to buy it.