Fonseca 1977 is a lovely port which is drinking very well now and will keep for a few decades if stored properly.
It is currently available from reliable retailers for around £130 per bottle. Selling it privately, such as to someone on this forum or a friend, or through an auction will not reach that price so you would probably be looking at £80-£90 per bottle, or perhaps a little less. That isn't because anyone would be ripping you off, it is because the £130 retail price includes £22 of VAT that you would not be charging but mostly because buying through an auction or privately represents a more risky transaction than buying through an established wine merchant.
From your follow-up question I assume you intend buying more Port with the proceeds of selling half of your case. My recommendation is not to sell as you are unlikely to buy anything at the same price that is as good or better than what you already have. I think a better plan would be to buy a few other bottles of vintage ports over a few months or a year to begin to build a collection that you can enjoy over many years. I am fairly certain that if you sell six of your Fonseca 1977s you will regret having done so on the day that you open the last of the six that you keep
Lastly, your garage is not a good place to store vintage port if you intend keeping it for many years as there will be massive seasonal and daily temperature variations that will cause it to mature rapidly and perhaps spoil completely. Stable temperature is the most important factor to consider. If you have a cupboard in the centre of your house (under the stairs?) that contains no heat sources that would probably be much better than the garage. If you can find a styrofoam bottle carrier you could use that to provide additional insulation that will help ensure that the wine stays at a more or less stable temperature.
Best of luck with whatever you decide to do with your bottles!