Should a buyer use the services of a realtor
when purchasing pre construction properties?

Yes, a realtor has relationships with developers that can be quite beneficial to the buyer. The realtor is also in a position to inform you when new pre construction opportunities arise before they are even made public. This means the buyer may have an exclusive opportunity to get in on the ground floor of pre construction prices. Your realtor will find out information about the pricing in similar buildings being built in the neighborhood along with information about civic and cultural developments. Furthermore the developer compensates the realtor, so in effect you are getting the services of the realtor for FREE. Since the purchase price of the property is the same with or without a realtor it makes sense to have a person who is not just a real estate agent advise you, but a real estate consultant.

Why Pre-sell?

Most developers are unwilling or unable to place all their funds into construction of a development that is not assured of success. Also most construction lenders (Banks) feel the same way and want to be assured the construction loans can be repaid in a timely manor. The answer for the developer is pre-selling the development. If for some reason it does not sell then the developer is out only a small investment. Pre-selling of buildings and resort condominiums has become a standard process, and virtually every condominium is pre-sold today. Once the required pre-sales have been reached and construction has begun it is a sure sign of a successful development.

You pick the building with the least amount of units. Why? For an investor it means less competition from other investors within the building looking to resell at the same time as you. You should also pick one of the first projects to be finished in the same price range as your building. Why? It is the same concept as above but on a larger scale. Other investors in other buildings in the area will also be looking to resell as soon as they can. By minimizing your competition you increase your profit and minimize your time in the property.

You pick the building with the least amount of units. Why? For an investor it means less competition from other investors within the building looking to resell at the same time as you. You should also pick one of the first projects to be finished in the same price range as your building. Why? It is the same concept as above but on a larger scale. Other investors in other buildings in the area will also be looking to resell as soon as they can. By minimizing your competition you increase your profit and minimize your time in the property.

That depends on your objective. If you are looking to resell for a profit as soon as possible the best unit to purchase is the least expensive on a low floor (view optional). Why? These units show the highest percentage of return based on previous resale figures thus having the widest margin of profit. On the other hand if you are looking to rent the unit to a tenant, you should consider a more expensive unit with a nice view. Why? Put yourself in the shoes of the renter. A renter expects a nice view in an area like Florida. Renters are on vacation and they want to see water, parks, boats, etc.

While I can't speak for every location in the country, here in Florida the purchaser of a pre-sale condominium is protected in several ways. The most important protection under Florida law is your money or deposit is held in escrow until the condominium is completed and only released at a proper real estate closing where the unit is transferred to you unless the developer specifically indicates in the contract that a portion on the deposits will be used for construction.

A Public Offering Statement includes condominium documents, association by-laws, and the pre-construction purchase and escrow agreement. Under Florida law, purchasers have fifteen (15) days from receipt to review and complete or rescind any pre-sale purchase and escrow agreement.

Two basic reasons:
(1) Limited location and availability.  
(2) By purchasing at pre-sale you are able to buy at a reduced price, and coupling this with the appreciative expectations upon completion makes it a very wise investment. 

Many people have purchased pre-sales for reason number 2, and many of these people have sold their units even before the building is completed.

With most pre-sale developments all you need is a 10% deposit of the pre-sale price at contract time.

In most cases, yes! You must have written permission from the developer, which is not a problem in most cases.
There are two basic ways to re-sell. (Depending on the developer) 
(1) Selling your rights to the condominium unit by transferring or assigning the contract to a subsequent purchaser. This in reality is not selling real estate instead you are selling contractual rights. In some cases profits may be smaller than the second option, but it has the same results! 
(2) Selling your unit under a real estate contract to be consummated upon completion of the condominium. Two real estate closings will occur simultaneously, your purchase, and the sale to the subsequent purchaser.

You have two options in most cases:
(1) The developer may resell the unit for you for a commission.
(2) The realtor may sell your unit for a commission.