Air shocks
 

I have nearly everything I need to build a small car trailer. Except metal.

On a trailer that weighs 250 pounds empty, and up to 750 pounds full.

So would 2 air shocks work as a air bag suspension for the trailer?

Re: Air shocks
 

IMO, shock mounts are not meant to carry a load. Definitely not for the sole purpose of suspension. Maybe if mounts are sufficient? Would not do it myself.
Bags are pretty cheap. Just get some and build the mounts if you want air suspension. The bags are probably cheaper than shocks and you could leave the shocks off the trailer.

Re: Air shocks
 

The thought was that air shocks are spacially efficient for what i had in mind.

So if they could lift up a few hundred pounds each. It would be awesome. Plus not have the issues with UV light air bags would

Re: Air shocks
 

I have had 3 different air shock setups. They have all leaked down slowly, despite taking much care with installing the lines. They can support weight.
I would worry about having them be the sole suspension, if there was a failure you'd be waiting for a tow truck.

In one instance the air shocks still held air as good as ever, but failed as shock absorbers after only a few thousand kilometers and were replaced with non air shocks to make the truck stable on highway again.

A pair would probably support a few hundred pounds easily. You could make beefy enough shock mounts.
But I would not do what you are contemplating, A couple leaf springs and straight axle makes a dandy trailer with no fuss. Remove some of the smaller leaves from the springs to soften the ride if you are building a light trailer.

Re: Air shocks
 

Just a thought I know you said you have all the parts to build the trailer except the metal.
But 30 years ago I got some great advice from the guy that welded up my utility trailer.
He advised me to use a 3500 lb axle because then you get the big bearings compared to the typical 1" bearings on a 2000 lb axle. I've had the trailer all over MT and ND with zero problems. Still got it.