Hi.
Please help, I know absolutely nothing. So need someone with a little patience.
Just bought a bike of ebay, the seller described as

The rear plastic part (front derailleur) on the gear is broken so no longer able to ride.

The chain is loose. I know which bits the front d and rear d is but how do I know which is broken? Can't find how to
upload a picture for some advice. Might not be a lot to go on but hope someone can help.
Thanks in advance

I'd assume the person knew front from back, but not bike terminology, so would guess he means that the rear derailleur is broken.

That's reinforced by the slack chain which is one possible symptom of a broken or missing rear derailleur, but not related to a front.

+1, I can't imagine a way that a front derailleur can cause a slack chain.

You didn't give your location so we can't make specific recommendations but see if there is a Bike-Coop or "Bike Kitchen" or a similar organization in your area that helps new and inexperienced owners with bike repair and has a wide variety of used parts available cheap. They should help you get you bike in rideable and safe condition at low cost.

I thought it was the rear, looking at it.(But what do I know?.. lol) as idiotic as this sounds what should.it look like. I wish I could upload a pic. Mine looks like, er....well, the little bit with the little cog bit on the looks like it should be in a certain position is all loose and upwards (Like 9 o clock when it should be at 7 o clock) if this makes sense.....
sorry again for lame terminology....
And thanks again for your patience. ..

You really need experienced help. Again, try to find a Bike Co-Op or a knowledgable friend to help you get this straightened out.

Post a link to the ebay advert.

I can only assume he is talking about the "jockey wheels".

your rear deraileur has 2 cogs that the chain snakes through. although it would be kind of a weird failure (the bolt sheared off and the cog fell out??) it is an easy fix if thats what the problem is.

To add some comfort I will say that even if the rear (or front) der is damaged beyond repair you can get a replacement fairly inexpensively. As long as the rest of the bike is what you want, a broken deraileur is not a show stopper.

Google Sheldon Brown and read everything about bikes you want to learn. You need to acquire some basic knowledge before you can proceed if you intend to do anything yourself. Also there is a sticky on here on how to post pictures somewhere. Use the search function to find it. You will eventually need to know all these things anyway so why not learn them first Happy cycling and welcome to the forum.

Even if you can figure out what's wrong you're going to end up needing parts, and a local bike shop would be the place to go for them. They can tell you how to put the parts in, or recommend you let them do it if it's a complicated job.