Firstly may I wish you luck with your venture
Secondly there are many costs you do not list I am afraid....I looked into this for an English friend about 18 months ago and due to the costs he decided it was a non-starter but here we go...
.importing items for sale from another EU destination requires registration as an Importer /Exporter and thus is a further cost( not to mention customs duty and if the UK votes out of the EU much higher customs charges)
Tax is charged around 20% up for self-employed ( and is payable whether you are making your estimated sales figures or not)however there is a low cost start scheme for Social Security(again only available to EU residents another loss if UK votes out)
You will require full residencia and NIF numbers ( again a grey area if UK leaves the EU)cost around 20€ if you can speak Spanish and do it yourself or budget around 80€ for someone to do it for you)
The waiting list for a stall at most of the bigger markets is around 12 months to 2 years in some cases ...can you afford to wait that long with no income ?
Most of the market traders and officials will only speak Spanish..can you communicate at this level??
You may be required to register for IGIC (local version of VAT if your ´´projected´´ sales exceed their limits ( more expense and payable in advance)
Markets usually only trade for a maximum of 4 or 5 hours (around 9 to 9.30 to 1300 or 1400) will you sell sufficient quantities of a single item to make it worth your while?
You say ´´homeware´´ so the tourist markets will not be much use( mainly ´´tat´´ and souvenirs...few real bargains and an awful lot of ´´copies ´´ etc ...so is your product truly unique?) and if selling to the locals you will need to be fluent in Spanish (Brits and Irish on their ´´jollies´´ do not tend to buy something that may come in useful once back home ! )
I am aware this seems to be negative but if your business plan does not cross the ´´t´´´s and dot the ´´i´´s then you may come unstuck very quickly,especially given the uncertainty of the forthcoming referendum.
Some markets will simply not allow you to sell an item if it is deemed that there are sufficient traders in the same product type......so have you any alternative if this is the case ?