Belts for treadle are relatively inexpensive (less than $10)
You can find an adjuster's manual online. They are really simple machines-doesn't take an expert. They were designed so that the housewife could keep them running in the old days.
I'd also offer less than the asking price. I bought my treadle for $50 a couple of months ago, you can find them on shopgoodwill.com pretty cheap and look at the offering for the ones in your area, you can probably just pick it up, no shipping fees.