The correctness of this phrase does not depend on the current numbers, as it refers to the types that the store sells in general. However, "sports car" should be plural in this construction, so:
The store sells all manner of sports cars, trucks, sedans, and vans. We currently have about 3 sports cars, 2 trucks, 5 sedans, and 3 vans. (I think the serial or 'oxford' comma improves clarity in this case.)
Although this is correct, this use of "all manner of" is in my opinion a bit old-fashioned, and not what would most often be used in such a context, at least in 2019 US English. The sentence could be recast as;
- The store sells many kinds of sports cars, trucks, sedans, and vans.
- The store sells various kinds of sports cars, trucks, sedans, and vans.
- The store sells all types of sports cars, trucks, sedans, and vans.
- The store sells a wide variety of sports cars, trucks, sedans, and vans.
Other options might also be chosen. The basic meaning of all of the above is much the same, and is also much the same as the original.