Well.. since he's talking about putting a hood on, i'd say it's a HOB
skimmer. Or he could be putting a skimmer meant for a sump inside of his aquarium.. which would not be a good thing.
First let me start by saying, for a 13 gallon tank.. you don't need a skimmer. You will accomplish what a skimmer does on a tank that size just by doing your weekly partial water changes. So if i were you, i would ditch the skimmer. Sorry that the guy at the store had to convince you to buy one, since you DO NOT need one. Just another classic case of a dirtbag store owner trying to make money off of unsuspecting people. If anything, i would try and return it to the store.
Next thing i would like to touch on.. is the fish and the size of your tank. 13 gallons is pretty small.. so be ready to only have about 1-2 fish in there. Anymore fish and you would be overstocked for sure. If you look at some of the threads around this site, in the nano section, you will see what happens to small tanks with lots of fish. Sure.. everything goes smooth with 4-5 fish in a 10G tank for a couple days, and then BAM, fish start dying, the tank starts to smell.. etc.. etc..
The point is.. it's up to you to do whatever it is you want, but we are just trying to help prevent you from spending money on fish/livestock that are destined to die in a tank that is just too small to have too many fish. Coral on the other hand.. the only overstocking point is when they begin to outgrow your tank lol, or they start killing eachother from the lack of space to grow out. :p