The Mollers Guide to 2st Tiddler Buying.
Scan the bloke's garage/shed for boaty bits.
Give the thing a good coat of looking at.
Shove it in a dustbin half full of water, get the yer man to hold tight and you give it a good tug (there's ya comp test).
Let the tell tale piss over his trousers and knock it in and out of gear. Try to avoid smacking the bottom out of the bin when in gear, I've done it.
If all good, offer him half of what he's asking.
Don't get the comp tester out, you may risk getting you're arse kicked down his drive.
Feel free to use this in the Ribnet FAQ section.