Not only what Dave wrote (that the voltage is likely to be too low),
but LOTS of other things too...
-- the RF losses of such a capacitor are often terrible
-- they are almost always polarized, not intended for use with an AC
signal but only as a filter for DC (especially one you'd pick up at a
flea market!)
-- I have some 1F 5.5V caps which are not so expensive new--but they
have very high series resistance. They are intended only as system
memory backup or similar for systems in which the backup current is low
microamps.
-- with such high capacitance, the loop would be so tiny that it would
be inefficient.
-- the self-resonant frequency of the capacitor is likly lower than the
frequency you want to tune the loop to. That means at the operating
frequency, the "capacitor" would look like an inductor and not tune the
loop anyway.
-- if you can manage to keep the losses low in the loop, even with a
reasonable tuning capacitance that you might actually be able to use,
the Q may be so high that the bandwidth is unuseably low.
-- the stability over temperature and time is terrible; the loop would
not stay tuned on one frequency.

These are broad generalizations. What you really need to do is look at
a SPECIFIC design and decide what capacitance makes sense from a system
performance standpoint. Will it be sensitive enough (as a receiving
loop)? Will it be efficient enough (as a transmitting loop)? Will the
bandwidth be large enough? Then the design will tell you what
capacitance you need, and you can ask the additional question: can I
tune it--how will I vary the tuning?

On very low frequencies, I can imagine using perhaps polypropylene caps
that are designed to work in switching power supplies and have low
inductance and low effective series resistance, but probably only for
use on one fixed frequency since changing the capacitance would be such
a hassle.

Assuming you're interested in low frequency receiving loops, have you
had a look at Reg's loop program? It can help you make decisions about
how to make your loop.

Cheers,
Tom