Thanks! So buying something expensive - like slaves - seems like it was pretty complex then: you couldn't just pop down to the market to buy a new gladiator and hand over the cash.

Slaves were res mancipi, which is to say items that were transferred by sale through mancipatio. Mancipatio was the oldest form of legal sale in Roman law, back when pretty much the only things that could be sold were livestock, slaves, and land. Accompanying any sale of res mancipi was an archaic ritual at which six Roman citizens had to be present--five witnesses and one dude to hold a pair of bronze scales. In the form of sale as described by the jurists the buyer would formally declare that he was now the owner and would hit the scales with a bronze coin, at which point the good for sale transferred legally into his power. This is the form of mancipatio as it exists around the middle of the Principate, however, where the single coin could symbolically stand in for the entire amount, which might be paid at some later date or on the spot, as agreed on by both parties. This was a pretty clumsy ritual though (and was clumsier still in its earlier forms), which is not surprising since res mancipi predated the Twelve Tables. The Twelve Tables also established a process of usucapio (holding through use), whereby possession of a good brought with it legal ownership after a period of a year or two, depending on what was being transferred. Usucapio is kind of weird, but the short version is that when it applied to res mancipi it stipulated that a res mancipi that was transferred only by traditio (handing over, which is basically to say sale that is not legally mancipatio and therefore with respect to res mancipi does not bring with it legal ownership, even though you really did buy the thing--that's a stupidly oversimplified version but it'll do for our purposes) and not by mancipatio became the property of the party to whom it was transferred after that one or two year period of continuous use. Res mancipi sold by someone other than the legal owner (i.e. an agent of some kind) was also sold as a traditio (provided the sale was done bona fide) and therefore legal ownership transferred to the buyer after the allotted period. There were various laws to prevent the abuse of usucapio by thieves and those who had acquired by force, and usucapio with regard to land was kind of weird (since land could not legally be stolen), but this was a very common means of acquiring ownership, and both usucapio and the evolved and symbolic version of mancipatio that existed in the Principate allowed for payment in full on the spot, or by other means as agreed upon by the parties of the sale, provided that the particular legal necessities of the ritual transfer of ownership were met