A contract requires an offer and an acceptance. If someone offers to sell something at a particular price, and you accept the offer and agree to buy the item at that price, that is a contract. For subject matter like camera equipment, it does not matter whether the agreement is in writing or only oral, emailed, etc. (For something like real estate, the contract likely does need to be in writing.)
Price is an essential term of a contract of this sort and if the parties have not agreed on a price, there is no contract but only ongoing negotiations.
Note that if the seller's offer is phrased as "first one to send me the price gets the item", then the only way to accept the offer is to send the money.
The legal remedy for a breach of contract is almost always money damages rather than forcing the party in breach to make good on his promises. In the typical RFF reneger situation, damages would be so minimal (e.g., the difference between what you would have paid for the reneger's summicron and what you ended up paying for a different one at KEH.com) that few rational aggrieved parties would pursue them.
Legalities aside, it's the easiest thing in the world to just not type and send an email to someone saying you will buy their equipment. If you say you are going to do something, you should do it. You should think through potential commitments before you make them, and be prepared to stick with those you decide to make. Otherwise, you will soon gain a reputation as a flake.