A couple of more tips:
1. plenty of detail and pics help, especially if there's a recent replacement of something significant.
2. expect requests for additional pictures of specific things.
3. a little humor in the ad can go a long way.
4. They say no reserve might inspire someone more to bid, but a reserve is safer and easier. The trick with a reserve is to not set it too high. I suggest using a reserve but with a very realistic bottom line price. This is why researching your car's value is so important.
5.. define the terms, like as is/where is, no warranty expressed or implied, how you'll accept payment (cash. crtified funds, paypal, pickup vs delivery, etc.
6. I agree about the no address/phone number in the ad, but your general location can be helpful, like "central NJ" for example.
as far as what buyers are looking for, difficult to say as everyone is different, and you neglected to mention the condition of your Fiat, like whether it's a project car, driver, parts car or garage queen, modified or stock (stock tends to fetch more than mods). the more information you provide the better. But if you're selling an old Fiat 124, rust is most likely the very first thing a buyer will ask you about, as rust came as standard equipment on 'em

, followed by the condition of the convertible top (I assume it's a spider), The top is a wonderful design, but they wear out more quickly if stored uncovered outdoors. Otherwise expect the same questions as any other 40 yr old car might get, maintenance records can be helpful. fwiw, I worked at a Fiat dealer back in the '70's. good luck.