Tam lived simply, on a farm that was isolated even by Two Rivers standards (and that's saying something) but that doesn't mean he was poor. Maybe he had gold and silver hoarded under the floorboards. It wouldn't have done him much good if he had; he might have been able to spend silver marks, but I doubt anyone in the village used, or would have had the money to give change to, gold. Money's not worth anything if people won't take it. And being the only rich man in a poor community is a good way to commit social suicide, so if he had money, he would have been careful not to flaunt it.

The question is more 'why did Tam choose to live in the middle of nowhere, if he could have bought a large farm, or set himself up in a town or city?' Put like that, it sounds more like a man who saw too much of war wanting to live in the quietest and most peaceful place he could think of.

Or you could make up any number of plausible reasons - he was robbed, or cheated, or gambled, or spent it all on a really expensive heron-mark sword. One of them will be true: the plot called for 'the hero' to be born a simple farmer, so that's how it was.